Level 2 "Gorge":  a very easy level.
Level 3 "Fiery Cavern": a straightforward level, with one advance lemming forging a path for both groups, while one group is held back, before they join up.
Level 4 "Path of Fire Twins": shows the advantages and disadvantages of Sliders. Needed a bit of tweaking to rescue all the Blockers, but not too difficult.
Level 5 "Frozen Pass": unless my solution is a backroute, some of the skills provided are red herrings. You need to know how to create an upwards path with Bashers, Miners and Builders.
Level 6 "Room of Shredder Traps": a step up in difficulty. 1 advance lemming needs to create a path for the others. That part is straightforward, but releasing a second advance one, some time after the first, is a challenge; the Slider plays a vital role. The solution needs perfect placement, both early on and near the end.
Level 7 "Swastika": Equally difficult. From the bottom trapdoors, the first lemming does the advance work; from the top trapdoor, the last does the key work. I had quite a few false starts with different ways of delaying the second lemming at the bottom left – most of which left problems elsewhere. One of my solutions uses the Stacker to delay; the other uses the Stacker in the obvious place, with a Miner doing the delaying.
Level 8 "SnowLem Land": a breather level. Needs the Blocker to turn a Builder, and careful attention to the topography to rescue the non-Climber from the right-hand group.   
Level 9 "Hard as Steel": an easier puzzle, but still needs a bit of figuring out to ensure that the 1 of 5 lemmings that cannot be made a Climber is delayed until a rescue platform is in place. In fact, only 1 Climber and 1 Miner are needed.
Level 10 "This Puzzle is Crystal Clear": I hate spider's webs, but even so it was not too difficult to save 99. However, care is needed to avoid the Hang Glider overshooting the exit. First I found a complicated solution, with the advance lemming bashing back to free the group, where I had to: time things so that the first lemming liberated by the advance one is the 100th spawner, which is only slightly ahead of the 1st (which follows close behind after looping round); increase the distance between 100th and 1st so that the 100th can be sufficiently advanced in its mining so that the early followers do not simply walk past the mineshaft; and save the Hang Glider, while still allowing the later lemmings to avoid the trap. Precise timing is the key to the first challenge. The spare Climber/Floater can help with the second. For the third challenge, a sneaky bit of pretend mining did the trick. But then I realised there is a simpler solution, which leaves 3 spare skills.
Level 11 "Center Point": Timing is very tight here. The first Stoner has to go in the middle. The second Climber is used only after the middle is reached.

With only 1 of 100 lemmings to be saved, and only 2 Bombers to get through 3 pit floors, a Nuke solution is clearly needed. So the final 2 lemmings have to reach the exit area before they explode, but they cannot access it until an earlier spawner has exploded. If I understand the game mechanics correctly, the time interval between the first and last explosions depends on how many lemmings there are between them, still alive at the point of Nuking. This creates a problem: if there are too many exploding lemmings, the ground at the bottom is blown away. So how to avoid this while still keeping most of the lemmings in play? I used a Builder and Blocker to corral the group to the right of the middle pit, while allowing the first spawner to escape. Timings and release rates have to be handled carefully. My final 2 lemmings were not tight together when approaching the pre-exit trap, but fortunately the trap is relatively generous.

Level 13 "Failed Hibernation": the traps are so rapid-fire that, for 1 of 5 Climbers to pass the 4 traps, the 5 need to be so tightly bunched that they appear on screen as a single lemming. Even this may not be enough, with the chameleon traps firing every 5 frames – far faster than a pair of Climbers can get through these traps. This made me think that bunching up the lemmings between a pit wall and a Stoner would not work. Eventually I realised that it's OK for each chameleon to gobble 2 lemmings; and that making the gap between a Stoner and a pit wall as narrow as possible is not a good idea. Needs perfect placement and timing, but a nice puzzle.
Level 14 "Salvage Operation": another challenging level. One key element for my solution is making sure that, when the group are freed, the first lemming to advance is a singleton, not one of the pairs.
Level 15 "Arrows": a breather level. Lemmings from the middle group free the left and right groups. I got the gold talisman on my second attempt.
Level 17 "Castle Conqueror": a good challenge, a large part of which is how to delay most lemmings while facilitating the necessary advance bridging. The 3 Shimmiers are used in 3 different ways, one of them a variant of a trick learnt in the first level.
Level 18 "Hall of Hope": not difficult to meet the save requirement and time limit, but my first attempt to do better ran into problems with running out of Platformers and using more Bombers than I wanted to. What was needed was an economical way of releasing at least 3 advance lemmings: the generous Climber allowance suggested a setup that allowed only Climbers to advance. Eventually I was able to save 74 lemmings (the requirement is 70) in a reasonable time.
Level 19 "Hydroelectric Powerplant": I learnt a new trick here, making a stack ascendable by using a Fencer, some distance away, to make a notch in the middle. Not too difficult if the skills are used frugally, and another generous time limit.
Level 20 "Watch Left And Right": an easy level to finish the rank, with lemmings from the right-hand trapdoor doing most of the work.

Simple use of Builders would leave the top of the bridge 3 pixels too low. So I would like to use a Shimmier with each of the first 3 Builders, and one at the end to stop it reversing. But there are not enough Shimmiers to allow this. So the final Builder reverses and has to be turned around, which costs me 9 seconds; and I miss the time limit by 4 seconds. Ideally the final Builder would be left with enough horizontal room to finish without then reversing; but if I do that the bridge does not gain enough vertical height. Still trying to figure this out.

Here I have no idea how to tackle this. With a locked rapid release rate, I don't see any way to stop several of the early lemmings from falling into the water. Perhaps there is some clever trick with Stoners that I do not know.

Level 1 "Oh Yeah More Lemmings": an easy introduction. It was straightforward to get the silver talisman. I have not attempted the others.
Level 2 "Season Round":  a long but not too difficult level. Getting the bronze talisman needed some careful thought.
Level 3 "Adventure of Pharoahs": another straightforward level, with a very generous time allowance.
Level 4 "Exit towards End": more of a challenge, with some tricky sequences needed to get the talismans.
Level 5"Lemmings in firework factory": it took me a while to work out how to use Platformers in different places to keep the crowd safe.
Level 6 "Dockyard": a nice puzzle. It's possible to save 49, compared to the required 47.
Level 7 "A lot at once lemmings": I struggled more with this than any other level in this rank. Use the Bomber and the Basher in the obvious locations, or swap them around? If a Builder is needed to rescue fallers, how to get this facing towards the exit, so the Builder does not head off and drown? Could using the Bomber in a 3rd location solve this problem? Eventually I realised that making full and proper use of the Hang-Glider is the key – though the solution needs precise timing in the early part of the level. Once the correct strategy is found, getting the talisman is easy.
Level 8 "Bottom of the well": a surprisingly easy level, needing just over a quarter of the time allowance.    
Level 9 "Re Energize your lemmings": another entertaining puzzle, with one of the two lemmings helping to rescue the other. Time allowance not so overly generous here, but still ample.
Level 10 "Going Up Lemmings": completely different from most levels in this rank, in that the required strategy is obvious but execution has to be pixel-perfect and frame-perfect. I beat the time limit by a tiny 0.06 seconds.
Level 11 "Crystals Cliff": another breather level. Here I looked for a strategy not using Bombers, so all 50 lemmings could be saved.
Level 12 "Think Fast Disarmer": not too difficult but still an enjoyable puzzle. 
Level 13 "Winter Rush": one of the more difficult levels in this rank. Do you get 1 lemming to the end, reverse it and bash back all the way? Or try to get past all the obstacles with the small number of skills provided? Or go for a hybrid approach? I reckon only one of these 3 options is viable. 
Level 14 "Twins Parkour": another entertaining 2-lemming level. One lemming has to wait while the terrain is prepared, but the time limit is not massively tight.
Level 15 "Donuts Paradise": I suspect there several alternative solutions, though mine has only 1 Builder to spare.   
Level 16 "Create a Paths": this took me a while to solve, until I realised that using the first Cloner in the right place would remove the problem I was facing towards the end of the level.
Level 17 "Gem Cathedral": here the talisman solution was completely different from my first effort – and with so many Builders to spare, I used some to speed up the later lemmings. Using the Digger in the right place was vital.
Level 18 "Caution Walk Over Bridge": a quick but entertaining challenge. It took me a few minutes to see where to use the Shimmier, because originally I wrongly thought it would be needed elsewhere.
Level 19 "Clone Wisely": one of the easiest levels in this rank, if you pay attention to the title. 
Level 20 "Miners and some dynamites": lots of Miners and Builders needed here. There must be quicker solutions than mine, but again I wanted to minimise the use of Bombers.

Like Kaywhyn, I found Tricky 3 "Which Way, Mr Scarecrow" the most difficult level – it took me a long time. Each lemming has to navigate 3 bi-directional splitters to get home, so – even ignoring the Zombies - there are 8 potential paths, of which 2 need no intervention, 4 need one, and 2 need two. So 8 lemmings would need 8 skills, and 20 need 20 – which leaves only 5 skills free for evading Zombies. I suspect that is impossible, so some lemmings need to be helped by either a Zombie or an earlier lemming turning the scarecrow in the desired direction. I looked for a pattern with the first splitter encountered: alternate left and right, or alternate in groups of 2, 3 or 4? Unfortunately I tried 3 of these options before finding the one that worked. With the help of 92Dexter11's tip about turning lemmings around, I eventually found a solution that uses just 23 skills.

Treacherous 3 "Pumping Station" initially looked like one of those levels where the required strategy is clear but execution is time-consuming and tricky – having to send up a succession of perfectly-timed Climbers to make sure that later-spawning lemmings do not splat. But it turns out there is a solution that avoids any risk of these splatting and only needs 2 Climbers (maybe only 1, but that would make timing very tight).

Treacherous 6 "Pushing Up Daisies" is ingeniously designed and provides quite a challenge for the Talisman. But it's one of those levels where you can get close and then do some final tweaking to avoid running out of Platformers.

Treacherous 10 "Ring Of Fire": a nice challenge to end on. I was having trouble rescuing the lemming that acquires the Jumper pick-up without causing problems elsewhere, so I looked to see if there was a solution not requiring the Jumper. And there is.

Fun: early levels provide useful introduction/refresher for skills used later on. I found a couple of possible backroutes, for levels 10 'Laugh if you like lemmings' and 25 'Lemmings, lemmings not everywhere'.

Tricky: Backroutes for levels 6 'Lemming psychology' and 26 'She has a clever plan'; maybe also for a couple of other levels.

Taxing: I particularly liked the puzzle provided by level 11 'The ascending shimmier scenario,' where  my solution is a minor variant of the author's, which saves a lot of time; also 28 'Poor lemmings.'  The hardest level for me was 26 'Trouble times 3' where I found it very difficult to synchronise the timings. My solutions for quite a few levels may be backroutes – notably level 2 'Many traps about'; level 12 'On the Edge'; 15 'This is an AWESOME level;' 16 'Heights beware'; and 25 'Follow the boss.' For level 7 'Every lemming must think for himself,' my solution is faster but far less fun. Level 30 'We might all fall down' is another level where a slight tweak to the solution can save a lot of time.

1 Blue Works: a lengthy but relatively straightforward level, with what seems to be a major backroute available.
2 This room is boiling: the most difficult level in the pack, for me. There are so many possibilities as to where to deploy the Cloners, and it is tempting to use the first one too early. Also, I wasted a lot of time trying complicated ways of accessing a couple of the pick-ups, because I failed to spot the easier way. The Zombies on this level are a minor distraction that just delay the effective start.
3 Be a hero: I reckoned there was only one way to solve this within the deadline, but I was wrong – my solution differs from the author's.
4 The roads that cross: 3 sets of lemmings to be co-ordinated. There must be lots of different solutions here; mine is reasonably quick. Whichever one you go for, you have to be patient with those on the right – possibly even the advance party. 
5 Up and down!: again there must be quite a few solutions available, at least in relation to accessing the pickup skills and creating a path for the upper group. 
6 This way or that: a relatively easy level for this rank, as I could not see any alternative solutions, apart from minor tweaks. Timing is tight though. 
7 Separated poles: not too difficult a level; again I would think there are several alternative solutions possible.
8 Don't be a rotten egg: timings are tight again here, but there's still some scope for tweaking the start and exactly how the Cloners are used. 
9 The Pharoahs curse: this is a very long level, in space and time. 3 trapdoors, 3 exits, buttons and teleporters make it quite a saga.
10 Hercules'pillars: not quite as difficult as it looks at first sight, but still a considerable challenge. I looked for a route that did not use so many Stackers.
11 We mostly all fall down: a breather level. With a locked release rate, there's no chance of beating the time limit by much. 
12 The side too far: another long level, with the advance party doing all the work.
13 The lemming thief: a nice little puzzle – only 2 lemmings, but lots of buttons to be pressed.
14 The split pea soup: a range of paths available, but good co-ordination needed whichever one you choose.
15 The quicky food solution: a close study of the skills available allows you to work out what the solution must be – I doubt if there are any alternatives to the main part of the solution. I ended up with just a fraction of a second to spare.
16 Only a minute: another easy breather level.
17 Don't step on stones: a good variety of solutions possible here, and an interesting use of a cascade of splitters. I had to work hard to find a way of speeding up the final lemming, to stop it lagging a long way behind the others. It's possible to beat the time limit by nearly 2 minutes, though that solution is not very elegant.
18 Only 4 of them: a very difficult level. In the end I came up with a crude solution, which meets the 'Save' target with over 1 minute to spare, but feels like a bit of a cheat. However, one of the pleasures of the game is seeing how many different solutions there can often be to any particular challenge.
19 Up you go!: one of the less interesting levels – straightforward, lots of platforming and bridging.
20 Discolored lemmings: it took me a while to work out how to access the pickup skill embedded in the ceiling. The vast array of traps before the exit means that the group cannot be released early. It's obvious where the Cloner has to be used, but there is scope for varying exactly how the Shimmiers are used.
21 A lemming twist please: a frustrating start, as you have to wait ages for the Neutrals to collect pickup skills. After that, it's an interesting challenge to work out where to deploy the Cloners.
22 A Beast!!! of a level: a key part of the challenge here is reaching all the buttons required to open the exit – quite a bit of trial and error is needed, along with some tweaks to stop a couple of lemmings heading away from the exit.
23 Up we go!: a very cleverly designed layout. Some aspects of the puzzle are easy to figure out, but I really struggled to find a way of freeing the lower group without causing a problem elsewhere.
24 All in please: fairly easy to work out what has to be done, but the timing is very tight.
25 Have a nice night!: the location of the teleporters and pickup skills gives useful clues to deploying the Cloners, and other key aspects of a solution are relatively easy to work out. But there is still scope for finding a speedy solution.
26 Steel areas of K*****: a key aspect of the challenge here is working out how best to access all the buttons, and in what order. How you use the Jumpers is important. A lot of trial and error got me to a speedy solution.
27 Another minute: a very easy level – maybe more appropriate to the Tricky rank? 
28 Don't mind the step: another long level; a bit of time can be saved if you find a way to avoid the Platformers reversing each time they bridge a gap.
29 Save me once more: I love a well-designed Zombie level, where you have to figure out a way of containing and/or removing the Zombies. My first attempt involved holding them back at the 1-way tower, but I couldn't get the timings right. In the end I came up with a tighter containment, which looks like a backroute. 
30 Meet in the middle: a nice puzzle to finish on. I could have done without the long wait for the Neutral to collect vital pickup skills. The layout and skillsets allow you to work out exactly where the Cloners must be used. But you have to work out how to use the teleporters; and it took me a while to realise where the Stoner and Stackers have to be deployed.

So did you find a way of getting the 'mine back all the way' solution to work? I spent a long time trying, but could not find a way.

The 3rd level, "Adding a Couple of Lems" was not straightforward. It's simple to save all but the last lemming; but freeing the Blocker without losing others needs the lemming that does this to be well separated from the others, and it took me multiple attempts to achieve this.
6th level "Basking in the Bricks" – why provide Disarmers in a level with no traps?
21st level "Jack of Crystals" – the key to making best use of the scarce skills is choosing where and how to use the Digger.
43rd level "The Cubical" - the solution to this has some nice touches, particularly in how to allow the advance lemming to get back to the group, and how to free the Blocker and get the advance lemming home.

1st level "Acknowledgements" – this level really frustrated me. From the terrain and the Save requirement, it's obvious what sort of solution is needed. But the terrain also means that each lemming lands in its first frame; this can make it difficult or impossible – depending on when and where a Blocker is positioned - to distinguish an individual lemming and assign it as a Floater, until and unless it falls into the pit. I ended up with a green tick here, but I'm not sure how, and it's not in my Replays.
11th level "Condensation" – requires quite a bit of thought to identify the complicated route which the advance lemming needs to take.
24th level "Modernised Level" – a nice puzzle, with the platforming needing careful positioning.
26th level "ProbLematic" – with careful attention to the terrain, this level is easier than it first appears.
28th level "Rainforest Hunt" – needs careful timing and positioning. At first I couldn't see the relevance of the Stoner, but this becomes clear later on in the level.
33rd level "Singing a Song" – this is one of several levels where a roundabout route is enforced for the advance lemming, in this case to help overcome the shortage of Bashers.
37th level "The Lem Stairwell" – another nice puzzle, where it took me a while to work out how to save one of the Climbers, which is needed to meet the Save requirement.
40th level "The Top Card" – this is the impossible level. I can't see any hope of a solution with the skills provided. I wondered whether there was a clever solution approaching the exit from the far side, but the terrain does not allow this.
44th level "Try the Farmland" – the 'Save 37' requirement can be exceeded, saving all 40. Maybe a backroute?
48th level "Wall Mutilation" – a straightforward level at first sight, but with a couple of subtleties. The terrain allows the Digger-Blocker trick if used at the right place. Also, you need to think ahead about how the Climber will be saved.
50th Level "Wish Comes True" – another tricky puzzle. Working out where to use the Digger and the Stacker was the key for me.

7th level "Ghostly Forest Beast" – the terrain here leaves many possible routes to ascend to the exit. It's not too difficult to exceed the target and save all.
9th level "Intruders in the Antarctic" – it took me quite a while to get this right; it needs accurate placement of the Bomber and the Miner. 
14th level "Silly Boy" – I'm not sure that my solution is the intended one.
15th level "Something Else Works" – this had me puzzled for some time. There may be several alternative solutions. The key elements for mine were: careful placement of the skills used by the exit, to give enough room for what needed to be done later; and solving the problem caused by this placement, when the mineshaft exit was just too high for walkers to ascend into it.
19th level "Too Far into the Novel" and 23rd level "Vacuum Sucker" – these levels are all about timing, to give the advance lemming(s) time to do their work before the followers come along and fall to their doom. "Vacuum Sucker" is the more difficult, but it's possible to save well over the required number.
24th level "Wafers in the Red Bricks" – I tried 3 different places to use the Bomber; one got close to a solution, but none of them quite worked. Then I realised there is a fourth option; with this it is possible – with a slight tweak - to save 39 rather than the required 38.
25th level "Walk in the Woods" – if my solution is the intended one, this level is a classic of misdirection. The problem with my first attempts was with the mineshaft: either it ended too high (at 7 pixels) for the group to ascend into it; or, if reduced to 5 pixels, this left a hole elsewhere in the mineshaft floor. The obvious likely remedy would be to place it to leave a step 6 pixels high – but, despite multiple tweaks, I could not achieve this. With a different solution 38 lemmings can be saved, but that's 1 short of the requirement. So I went for something completely different, where it's easy to save 38, and the required 39 can be saved with a bit of care; one feature of the terrain makes me think this is the intended solution, though I'm not completely sure.

Early on you should be able to lose only 2 lemmings as you platform.

After that, the solution relies on one of those Neolemmix tricks of the trade, where something that arguably should not be possible actually is. I won't be more specific now, as that would give the game away.