The lightbulb moment with this level for me was realizing that the climbers needed to be closer together so that the second climber is already past the point where the bomb impacts the wall he's climbing.

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.

You clearly have to use all 5 builders to cross the water, which only leaves you 5 diggers to get most of the crowd down safely. If I try an AT style solution to turn them around at the far end, hardly any of them get back up before the digger steps become too high to climb. Conversely, if I use the old stair steps technique of creating 1-pixel offsets by using one digger to cancel the other, most of the crowd end up going towards the fire trap, where they all obviously die. It's not really the kind of puzzle where you can come up with multiple ideas of things to try, I find, and I'm pretty much out of ideas already!