Black Hole 1 -
Thin Layers Ok, I was probably the biggest dummy here, as it apparently took me 3 years to get this solved! The solution isn't that difficult once you do figure it out, but there was one spot that I failed to realize for 3 years, which is the junction in the upper left where you can bash with the climber, who then goes back to the right to faller bomb the wall away for the others. Before this, I kept doing everything from the right side, leaving me a basher for the end, but to my annoyance and chagrin, at the very top, this makes everyone fall a pixel before the pole next to the exit

Thus began the very long struggle on this level. There's also a lot of red herrings that look like they could work but they don't. Thus, the solution is very well-hidden, but it's in plain sight.
Even though I managed to finally get the level solved without hints after almost 3 years, I'm not necessarily proud of this. It's a great level and all, but I can't believe how dumb I was this entire time

Black Hole 2 -
Lament of Innocence I originally solved this with the very fiddly basher/blocker trick where the basher turns around but the blocker still gets released, at a time when I got frustrated with not being able to figure out Black Hole 1 and hence I was curious about the next level. So, when I finally did reach this level for real after getting Black Hole 1 solved, I decided to look for another way to solve the level without the very fiddly trick. No luck, but I was pretty certain it is possible. Just that my near successful attempts would get the climber into danger. So, I slept it off for about 2 hours or so, especially as I was quite tired from not enough sleep from the night before, but shortly after I closed my eyes, I realized it was just simply interrupt the miner with a builder shortly after he makes a hole for the climber to get up and not fall back down and splat. Not a very hard level, but still somewhat deceptively challenging.
Black Hole 3 -
The Rule of the One In contrast to the previous two levels, I solved this one very quickly. It is a bit fiddly, but as there's a long walkway it delays them enough to allow the floater enough time to bash and build to the exit. It even saves 1 over the requirement

Black Hole 4 -
Gray Walls Another very quick solve here. Looks difficult, but it really wasn't that bad at all. Even though I found this easy, I can still see this one being difficult, as it's not easy to figure out the best way to navigate the level and construct the route.
Black Hole 5 -
Cracked Stones I found this one somewhat challenging but a pretty good one. it took me about a good half hour or so. I realized what the intended solution must be while trying something that I think would work but wasn't as certain about. Indeed, it's quite fiddly and involves RR tweaking. I'm not even sure why I thought it would work, the miner taking a stroke at the right time when the climber comes so that he turns around but the miner still makes a hole for him to fall out to the left. So, it's nice to know that it is indeed possible, but again it's very fiddly. I pretty much had most of the level figured out, with the hardest part being to figure out how to get a climber to mine out the crowd. It's also hard to see who is the sacrifice here.
Black Hole 6 -
Deep Frozen Misery Harder than I thought it would be. I figured out how to make the splatforms and all, but I would always be a lemming short. Checking Icho's solution, it seems that I overcomplicated the solution a little bit.
Black Hole 7 -
The Everclimbing Lemming Sorry Nepster, backrouted this

I still found this one challenging.
Black Hole 8 -
Moonshine Trio The hardest part is certainly figuring out how to rescue one of the blockers at the very top. Indeed, this was somewhat of a quick solve, but it still took me time to realize that the left blocker needs to be near the digger tunnel so that the climber will climb up, turn around and then before he falls back down you need to make him dig so that he releases the blocker. The other hard part is making the builder and miner cross at about the same time. Nice level, though!
Black Hole 9 -
Mining Company Very hard level but an excellent level and a favorite of mine!

It looks impossible on first glance, especially with the high RR, but it's all about knowing how to use the miners. The trick of using a second miner so that no one can go into the first one isn't new to me, as I learned this from playing United. It's just a bit hard to use because of the somewhat small platform size. Very difficult timing. Well done here, Nepster!

Black Hole 10 -
Ploughing the Fields Easy one, but again I can still see this one being challenging for less experienced players. It's all about knowing how to stall the crowd long enough to give the worker enough time to build over the gaps and then make a path to the exit before they catch up. I really like the making a pit on the far right first and then bash towards the entrance, and then another digger pit close to the entrance in order to stop the basher going to the left. This one needs to go lower so that you can bash all the way through to the right.
Black Hole 11 -
Woodworld I found this one very hard. The solution came to me after about 20 minutes or so, but it was all about working out the timing so that no one gets into danger. Mine is quite fiddly and a bit more complicated than it needs to be. Nice level nevertheless! I especially like the digging with the first climber and then the second climber is able to bash all the way to the left so that you can get through multiple obstacles with just one skill. Any other way would leave you at least a skill short.
Black Hole 12 -
9/11 Somewhat hard, but it seems I solved the level unintended for the most part, so it's probably a backroute backroute but it's likely still acceptable as I still use all the skills. The second entrance from the left is certainly the hardest to manage as they're the only ones headed towards danger while the others are already contained.
Black Hole 13 -
Another Generic Title Somewhat hard as well, though this one was fairly quick in terms of solving time. In one of my earlier attempts, I wouldn't be able to isolate a lemming far enough ahead of the others to build to the exit before anyone slipped by. Then I found a way to do so with absolutely no timing needed. Great level here!
Black Hole 14 -
Bashing & Building Nice level, though once again this trick of interrupting the builder at the right time with a basher isn't new to me, as I learned this from United as well. I suspect that this level might had taken me longer had I not learned this trick elsewhere first. However, as my experience shows, it still wasn't automatic even though I learned the trick from another level pack. It still took me a good 20 minutes before I realized this trick is needed and solving the level.
Reading that this took Icho a week surprises me, as he's pretty much like the best solver there is in this Lemmings community, but it does make sense. The reason is simple: The trick was new to him at the time, therefore that's why he got stumped here for a long time.
Black Hole 15 -
HangLem I would probably consider this the hardest level in the rank. It's not clear how to achieve the compression needed to be able to get through the traps so that only one dies to each one. It's obvious you need 8 of them clumped up enough for the lucky one to reach the exit, but how to do so is the hardest to figure out. My solution is another very fiddly one, and I even managed it with 9 running the gauntlet and without using the digger. This probably means it is possible to save 2, but I'm not certain. Even then, I'm not too much of a fan of this level as well due to the fiddlyness.
Black Hole 16 -
Devil's Right Hand Harder than it looks, but it's still not very hard. My earlier attempts had me max out the RR and use the last two to get down the middle to make a splatform and build over the gap, but when I bash through the OWW on the right, at least one kept turning around since he didn't get through the wall yet. Instead, it was hard to see to make the splatform at the bottom on the far right. I also built over the bomber hole giving the builder at the bottom enough time to make the splatform. Nice level nevertheless!
Black Hole 17 -
Fall and never Live! A bit hard but once again not super hard. Yet another fiddly solution I came up with

The hardest part for me was figuring out how to stop the athlete from going to the exit before he's had a chance to make another splatform near the bottom, but it was just a simple non-athlete blocker to do so.
Black Hole 18 -
Lemming Mayhem Somewhat easy but a great level. The hardest part of the solution to see here is maxing out the RR to get one past the trap. The rest of the level is easy.
Black Hole 19 -
One minute of action Another backroute, I'm afraid

Not a very hard level but not a trivial one either.
Black Hole 20 -
The Deadly Rooms of Lem Even though I use all the skills here, it might still be a backroute? My solution is very different from Icho's. This one is definitely harder than either of the previous two levels.
Black Hole 21 -
Final Frustration To round off the pack is a very wide 20-of-everything level. This is definitely one of the hardest X-of-everything levels I have ever played, especially since you need to save everyone on top of it all! It's extremely difficult to find the most efficient route that wouldn't use too many skills in any area. I came quite close in some of my earlier attempts, only to find that I've ran out of a very useful skill and therefore wouldn't be able to make it a solve. I especially found the builders the most problematic to optimize. Really great challenging level to finish the rank and the pack!
For the record, I've played several Final Frustration type levels before solving this one. In particular, one from Gigalem's ghost pack, the final level of Strato's LWT, and the final main level of GGF.