Level 12 "Offerings Could Save Others": I delayed taking a close look at this level, for fear that it might be a miniature version of level 10's Digger-fest. But actually it presents a much more interesting challenge. In particular it needs some tricky choreography near the exit, where 3 different activities need to be synchronised.
Level 15 "Scorching Terminal": it becomes clear that: (a) the advance lemmings have to take the low road; (b) the crowd have to take the high road. The problem is that each road ideally needs a Cloner, but only 1 Cloner is available. For (a) the required path is relatively easy to see but executing this without using excessive skills or the Cloner is very difficult. The intended solution apparently requires 2 special tricks, and I know the one involving the Cloner, but not the other one so I fall 1 short of the Save requirement.
Level 16 "Gloomy Night": the hairtrigger chameleon traps, triggering every 5 frames, make it enormously difficult to meet the save requirement. It becomes clear that evading 2 of the 5 traps on the obvious route to the exit will not be enough, even with a tightly packed crowd. At least 1 of the 2 traps nearest the trapdoor needs to be bypassed completely. With no Diggers or Miners, the only way to bypass from above is with a Platformer - otherwise a way is needed to use Bashers or Fencers acting from below. For a long time my best efforts, using Glider-Shimmier to get the advance lemming over the gap on the right, and platforming above 1 of these 2 traps, still left me 2 short. There is a route that bypasses both the near-trapdoor traps and gets the required number to near the exit, but with no way of accessing the exit. So I looked for a 'from below' approach using the Shimmier in a more conventional way. This took me to a solution which, in all its key components, is the same as IchoTolot's. This approach succeeds only if the crowd are released in exactly the right way - I got lucky at the first attempt, then failed in my next 4 attempts to recreate the approach.
Level 19 "Decimation": trial runs show that, within the time limit, it is impossible to save the final 8 or so lemmings via the right-hand exit, using the conventional route; and that they cannot be saved via the left exit unless an advance lemming clears the way first. I cannot claim much, if any, credit for my solution, as it draws on the first 5 seconds of IchoTolot's solution and the use of the Miner in MASTER88's 'Common Mistakes' YouTube video showing unsuccessful solutions.
Level 20 "Havoc Highway": my additional solution saves all 103 lemmings. Inevitably this takes a bit longer than just meeting the Save requirement, and needs a few more skills; but there are still 6 skills and over 6 minutes to spare.