Here is how I do it:
1. A0;Make the third and fourth lemmings float as soon as they appear from the entrance. A0;(I'm not sure this is completely necessary, but it gives the two front lemmings some extra breathing room.)
2. A0;Make the leader build just before he gets to the stairs. A0;He should turn around. Then make him build on the left edge of the platform.
3. A0;Make the first lemming on the stairs dig as soon as he reaches the third step from the top. After one swipe, make him build.
4. A0;If a lemming falls through the digger's hole, make him float. A0;If not, make somebody climb + float. A0;This lemming will be our hero.
5. A0;At some point the builder on the left will run out of bricks. A0;Make him build again as soon as he does. A0;Hopefully, he will reach the wall before the rest of the crowd reaches him.
6. A0;The hero, who should have floated to the bottom of the screen, must build across the gap just past the top of the stairs on the right, turn around, and build off the left side of the platform at the bottom of the stairs toward the first set of stairs (it takes 3-4 bridges).
7. A0;The hero should bash and build to complete the route to the exit. A0;Meanwhile, the front lemming from the crowd must mine through the stairs and land on the hero's bridge. A0;If you were very lucky (it usually takes me about 5 tries to get it right), you will save 100%.