Interesting idea, although I'm not sure how many people here design levels in this sort of mindset. I know I do, but then I don't think very visually, and as you can see, as a result I haven't made many levels.
Anyway, here's my answer to your challenge. The attached ZIP file contains the LVL as well as a Lemmix replay, which is made opening the level in "Custom Lemmings" style (but the exact same solution should work on the level unmodified with DOS Lemmings mechanics). Even if you can't watch the replay, it should be blatantly obvious what the solution is anyway.
I'm too lazy to think of the next one right now, so I'll just call "open floor" until I got something or someone else beats me to it.
[edit: fixed level so that all non-floaters will exit, as required by challenge]