1) Make the first lemming build at the edge.
2) Make the first three lemmings(ie: the builder and the next two) into floaters.
3) Make the builder block after taking a step after finishing. (It will not work if he doesn't take the step first)
4) Make one of the next two lemmings mine just a bit before turning on the blocker. If done right, he will turn and fall through, yet other lemmings will turn as normal in this place.
5) Make the faller into a floater.
6) Make him build off the bottom bridge to the left.
7) Play with the release rate so that you get two lemmings fairly spaced apart from each other and more importantly from any others. I find setting it to 60 just before one comes out from the left, then back down to 1 right after he does, works quite well.
8) Make the first of these two lemmings into a floater.
9) Make the first one mine as close to the blocker before turning as possible. If done right, this will free the blocker.
10) Make the second one mine at the foot of the bridge the first lemming of the level built. This is because the fall is not survivable even now from the top.