Level 8 "Lemmings Are Grounded": not yet solved. I am assuming that the solution involves an advance lemming going along the top, mining to the right and then being turned by a Blocker which climbs up the wall, then a Basher to free the Blocker & get the group to the exit.
Level 9 "All Stepping Stones Don't Help": I had problems with this because of a false assumption on my part. I thought that Sliders would splat if they descended from a platform, where the distance from the underside of the platform to the landing ground is 3 pixels more than the distance from the bottom of the splat ruler to the pink mark. But they don't - even when the Slider's lower foot (in the final frame before it transitions to a faller) is 1 pixel higher than the pink mark, with the splat ruler anchored on the ground. Once I realised this – thanks to a sneak preview of part of Master88's solution – it just needed some experimentation to find a solution. As Master88 says, there is more than one possible solution.

Level 11 "Turbo Tunnel": this level is all about meeting the time limit. A trial run shows that it is not possible for the last lemmings to reach the lower exit within the time limit. From this it follows that they must aim for the other exit; and this dictates how 3 of the skills must be assigned. The remaining challenge is to ensure that those lemmings that are reversed while obstacles are being removed are themselves halted and re-reversed in good time. Timing is very tight – I had less than a second to spare – but there is ample scope for tweaking the timing and positioning of the manoeuvre that delivers the re-reversing in good time. 
Level 12 "Season Of Miners": I found it relatively straightforward to save all but 1 lemmings. The problem arises at the end, where advance lemmings - that mine back away from the exit - have to be halted and reversed. It took me some time to find a workable option for removing the Blocker – requiring careful attention to detail to economise on miners. Timing not so tight here.
Level 13 "Walking On Hot Coals": an amazingly easy level, solved with quite a few skills to spare. Presumably a backroute.
Level 14 "Methods Of Masters": not yet solved. It is easy to see where the Cloner should be used to help create a route for a Basher to get through both central blocks – but very difficult to implement.
Level 15 "Final Trick": another level where you can easily fall 1 short of the save requirement. Looks like it needs a clever trick with the 2 Platformers, but I don't know a trick that helps here.
Level 16 "There is No Time To Waste": looks impossible within the time limit. The Digger above the exit has to stop at a particular point: any sooner, the crowd will splat; any later, the Digger will exceed the time limit. This restricts options elsewhere. If this Digger is advanced by making it a HangGlider, it will miss the exit. Otherwise, the crowd have to be compressed and then released so they reach the above-exit Digger just as it is halted, to avoid the crowd either splatting or exceeding the time limit. The best I can do so far is to rescue 37 out of 40 using 5 skills, or (sacrificing the Digger/Glider) 38 using 7 skills.
Level 17 "Edge Of Steep Cliff": it took me a while to spot the obvious (or should-have-been-obvious) solution to the problem of how to hold back the crowd while advance lemming(s) prepare a landing ground. And then I could not get the 'reverse a Builder by building just beyond another' trick to work so had to find another approach. Ended up comfortably within the time limit and with plenty of Builders to spare.
Level 18 "Puzzle Of Pharoahs": getting the group on the right to the exit is fairly simple, but it's more complicated for the left group. There are Miners and Blockers to spare, but careful thought is needed for where to use the Builders. The time limit is not an issue here.
Level 19 "Polar Night": At my first attempt I ran out of Miners and Platformers way too soon – partly because there was one obstacle that seemed to need quite a few of these. But there is a way round that. One advance lemming has to do all the work. And with one of the two groups you have to sacrifice some skills to allow its release only after a delay.
Level 20 "Hour Glass": a Zombie level with only 2 active lemmings, but the key challenge is to reach all 11 buttons – 1 guarded by the Zombie – before the Neutrals arrive at the exit. This is really tough. On the left, 6 buttons make for 720 possible sequences; on the right, 5 buttons and the Zombie guard on one make for 360. Many of these can be ruled out immediately, but that still leaves quite a few potentially viable options. Timing is fairly tight, as is the supply of Builders, so every effort is needed to economise on both; and the zombie climber/floater needs to be removed from the scene. A nice challenge to finish the rank with, though I reckon level 16 is harder.