This level can be highly confusing, the high save percentage won't help either. Thankfully, it is not as hard as it looks.
Allow the first Lemming to walk a little to the right, then set him as a Blocker. Allow the second Lemming to wander to the left, and the third Lemming should also be set up as a Blocker, so they'll keep the crowd at bay. Your sole Lemming must use a Builder skill above the gap in the stainless steel floor. The drop may or may not be fatal, but why take chances - build up another ramp toward the wall, where its slant is high enough so your Lemming won't break his neck and land safely. Give him Basher ability at the one-way rock, and make him a Builder at the chasm. While doing so, blow up the left Blocker to let the crowd loose. Once all your Lemmings have reached safety, you can detonate the remaining Blocker.