Max level size: 3200 x 3200
Release Rate: 1-99 (locked RR supported)
Time limits (<= 99 minutes, infinite time supported)
Max lem count: 999
Normal lems only (no zombies or neutrals)
Classic 8 skillset only (<=99 thereof, infinite skills supported)

Available objects:

Fire
Water
Continuous traps (i.e. one-shot is not supported)
One-Way-Arrows (all directions are supported)
Force field left/right (essentially the Blocker effect)

Level 1 "Room of Levitation": this caused me a lot of trouble. Platform from the bottom, climb and shimmy at the far right, then dig and somehow find a way to get the group up to the exit? Or platform from the top and get over the traps? And how to hold back the masses without using too many skills? My solution to the second issue uses 2 Shimmiers and 2 Stoners; and leaves 3 skills spare, so maybe it's a backroute.
Level 2 "Deep Dark Mineshaft": with so few skills and options available, it should be easy to find the solution by a process of elimination. But the second advance lemming has to be timed just right to reverse the first and get in the right place for a faller from the stone to Bomb the trap without removing too much of the bridge.
Level 3 "Windy Well": A way down is needed for those lemmings from the top trapdoor. But with only 1 Miner, and no Blockers, most of the work has to come from below. There are 2 challenges here. One lemming from the bottom trapdoor has to be really economical in its use of Fencers and Stackers. And at the top, the bridging needed for all but the first 5 spawners prevents the early ones from reaching the shaft, which calls for an alternative way of getting them home.
Level 4 "Diamond Mine": the challenge here is to get 2 Climbers close together – but not too close – so one can be made a Stoner which becomes a platform for the other. The set-up makes this tricky to achieve within the time limit. I ended up with 8 seconds to spare.
Level 5 "Diabolical Run": a breather level. Trial and error identified which lemming had to be made the Digger so that the first could advance well ahead of the others.
Level 6 "Under The earth": this was tough. I thought I'd found a clever Builder-Stoner-Shimmier-Slider trick with the advance lemming, but then realised that being a Slider took that lemming to its doom. So it had to be repurposed and the trick replaced by something simpler.
Level 7 "Sacfrife": it took me a while to ensure that, of the lemmings rescued from the left pit, not a single one goes off to its doom on the left. In the end the solution was easier than I expected. BTW, not sure what the title means – Sacrifice?
Level 8 "Triple Trouble": skills to spare here. Initially I over-complicated things looking for a clever way of rescuing the bottom lemming that can't be made a Swimmer, when actually there is a simple way.
Level 9 "Rampage In Fossil Canyon": a level I struggled with. It's easy to save all but the Blocker, and set up the Blocker to head in the right direction once released. But saving the Blocker needs 4 of the 8 terrain-removing skills, and a Basher is also needed at the top. That leaves only 3 terrain-removing skills to get the crowd up to the bridge; frame-perfect precision and a bit of lateral thinking are needed for this.
Level 10 "ACT FAST!": relatively simple. I managed to save 1 more than the requirement, with a massive 0.24 seconds to spare.
Level 11 "Capital City Web Ville": I usually struggle with webs. This time, I had to decide whether to use one of the Shimmiers to halt a Miner, or use it to follow the first advance lemming – which turned out to be the right option. The Sliders come into play by allowing the advance pair to Bomb in exactly the right places. Then Miners or Diggers are used on the left side of the web to allow a handful to reach the right side where they can help free the group. Once this is worked out, it is straightforward to save 76 and get the talisman. And with a different approach to containing the crowd at the start, 77 can be saved.
Level 12 "Mole Miles": another breather level, and easy to get the talisman.
Level 13 "Extra Steps is Accetable": options are limited here, so it does not take too long to rule out some options and find the one that works.
Level 14 "Meeting Point": by far the most annoying level of the rank, for me. The strategy is clear, but the execution very difficult, with one Digger having to be cancelled by another, timed so the pit is deep enough that the 4th lemming cannot go to the right but not so deep that the crowd falls into the water. It took me ages to find the right combination of release rates, Digger selection and location, and Platformer timing. Though at least there was scope for some calculations to find the combination. Later on, a Basher-Platformer from the right and Basher from the left have to be synchronised perfectly, and this is purely hit and miss – the Platformer's final tile kept on being removed by the Basher from the left, even after that Basher had finished and become a Walker; why is a mystery.
Level 15 "Lagoon of Erruption": it took me a while to decide what route was best for the lemmings on the right. For those on the left, the issue was how to release 3 advance lemmings, suitably timed.
Level 16 "Boiling Climb": I enjoyed this level. Stoners have to be used in 4 different ways: to contain; to reverse, with a Slider; to redirect a Shimmier; and to bridge a gap.
Level 17 "Lemmings Dream Wonderland": another example of the required strategy being clear – because no alternative could be viable – but the execution difficult. It took me ages to achieve the pixel-perfect precision needed for this.
Level 18 "Rules of Freefall": as with quite a few of Master-88's levels, it pays to look very closely at the terrain. The Builder does more than just bridge the gap. I did not find a use for the Climber.
Level 19 "Hot As Loaf": another tough one, as you'd expect from the rank position. The first challenge was how to use the Slider and Shimmiers economically, via the Cloners, to forge an advance path. The second was to ensure the crowd all head left on exiting the shaft – easy enough to arrange with a Builder over the shaft so they can only enter the shaft from the right, but not so easy when that Builder is needed elsewhere.
Level 20 "Frigit Crusade": an enjoyable challenge to finish the rank. Also a very honest puzzle in the sense that the skills provided, plus careful consideration of the opportunity provided by the terrain, lead you to the solution, enabled by precise timing of Shimmiers exiting the pit.