Level 1 "Jungle Speedway": the chameleon trap by the exit means the crowd have to be compressed very tightly. I have several skills unused, but that does not necessarily indicate a backroute..
Level 2 "Sfift Action Freeway": I tried 2 routes, coming up 1 skill short with both. A 3rd route I'd been considering from the start looked impossible to execute, but then I realised that using one of MASTER-88's favourite tricks would make it easy to achieve the necessary synchronisation between the advance lemming and another. There are several viable variants to my solution – for example, in two variants I don't clone the Disarmer, and in one of these I save 100 rather than the required 99.
Level 3 "Lethal Sidewalk": a relatively straightforward level, with very generous time limit.
Level 4 "You Have To Take Little Detour": I had 1 false start with this. You need to know one Stoner trick and be very precise with positioning and timing.
Level 5 "Minute Mile": mind-boggling. The key challenge here, besides the super-tight time limit, is how to get past 2 obstacles while sacrificing only 1 lemming, since 1 has to be sacrificed by the trapdoor. There is no way past the 1-way pole without using a Stoner or a Bomber. This means the Flipper has to be passed without losing a lemming. I looked at various Blocker-Flipper interactions, but none of them quite worked, within the time limit – leaving me with several ways of rescuing 77 lemmings, but not the required 78. Eventually I realised it was possible to free up a skill and use it in a weird way that indirectly allowed the Blocker-Flipper interaction to deliver as required. If this is the intended solution, the level design is so ingenious.
Level 6 "Demolition in Coalmines": this is a large and long level, with a 15 minute time allowance. It is very difficult to free the crowd without using too many skills. I considered freeing them from below, using the sole Basher to go underneath the trapdoor chamber; but this route faces major problems with the chains and firebreaks. My solution, by using spare skills to reduce delays, takes well under 10 minutes.
Level 7 "Lemmings in Slaughter House": at first glance this does not appear extremely difficult. There are 2 Shimmiers to help get the advance lemming to where it needs to be to forge a path for the crowd. But then you realise there's a major problem with the group on the right: 2 traps to disarm, plus a high release rate, spell trouble. Solving this headache needs lateral thinking and experimentation. Another challenging puzzle.
Level 8 "Bullet Train Lemmings": another great challenge. I looked for a way to shift the first lemming from the front of the crowd, where it has to be to disarm the first trap, to the back - to give it ample time to disarm 2 traps and mine to the exit. I think this is essential to any solution. There are some spare skills that, as far as I can see, cannot be used.
Level 11 "Lemmings On Thin Ice": a much needed breather level, where it is not too difficult to beat the save requirement and save all 80. The time limit is very generous.
Level 13 "Very Tricky Position": this needs precise positioning of the Builders, which took me several attempts; and some very difficult work at the end, using the same trick as in level 15 of the 4th rank.  I don't see why the time limit is twice what it needs to be – maybe an element of misdirection?
Level 14 "High Place Expresslane": the key to this level is finding a mining route that is economical on Builders; and the key to that is where the Bashers are deployed. Working this out is challenging, but once that has been achieved it's not too difficult to meet – and exceed – the requirements for the 2 Talismans.