You're supposed to use some weird Lemmings behavior, dealing with hmm, I'll be a little vague, but climbers and/or floaters. It requires a pixel-perfect move, but only one. I guess I could just make that trap a pit...
Seems like everyone has passed all the levels in this pack using backroutes (except I guess levels 1 and 2... I'm pretty sure a backroute for 2 exists though, but I forget what it is), which I'm not really surprised about. Sometimes fixing a backroute makes the intended solution stand out a bit more, though, and I'm not sure I want that either. But I'll probably make an at least partially fixed version of this pack sometime soon...