Wow, this one was a very hard one for me. I solved the other 3 much quicker, but I was stuck here for months! It turns out that I did a lot of things incorrectly. For example, many of my attempts had me going through the bottom on the right side. However, this would cause a lot of problems getting through the area with the fire trap, where it is a bit visually deceptive. Also, by doing this you will always be at least a skill short. I just kept trying this, but no matter what I couldn't get it solved. For example, two stackers is enough to get a lemming to jump to the part with the fire trap, but then you don't have anything to turn the climber around due to no more stackers available.
Figuring out how to get the lone Lemming to the other side is hard, and also getting up to the exit as well! I also hated how a lot of things kept failing by at least a single pixel

For example, platforming at the beginning and jumping doesn't work, as the Lemming does not even land on terrain, as it is short by a pixel. That being said, the platforming and then jumping to land on the musical note at the bottom is very precise. I would definitely reduce the precision there, especially since it's not very clear that it will work.
I then just started playing around by having the lone Lemming go through the top of the instrument and seeing if I could do other parts more efficiently later on. My breakthrough was when I tried bashing and then cancel him midstroke with a walker and see that he can step up out of the tunnel to be on top. I think the rest of the solution came easily after that. I would also say it's very hard to see that 3 builders is enough to get to the exit, at the place where you mine the crowd out.
But yes, I got to say this was the hardest level for me out of the other 3. It was mostly due to how I couldn't figure out how to use the walkers for a very long time. I thought they were used to cancel a stacker before it got too high for the others to step up. Very nice level, but a bit too heavy on the precision for my liking. Very well hidden solution.