Ok, I've now come to an absolute halt on the Genesis version of Lemmings. I must have less than 20 levels left (and I know I can do some of the later ones) but in terms of doing it properly and not skipping levels, I really am stuck now.
Present Level 29 - Private Room Available
The 40 diggers seem to suggest that you need to dig a lot. The only way Lemmings can survive the drop is if they dig on one of the threads dangling off the boxes. If they don't, then they die from the fall. But even grouping up Lemmings I can't see how to get it to work because the thread hanging off is so thin that some Lemmings will fall further and die when the digger breaks through the floor.
Present Level 30 - Final Impediment
I'm not there yet, but omgwtf! I think this is a case of a Hero Lemming where you contain the rest and let one Lemming do the work. I can contain everyone and have one Lemming go off on his own, but the flames bursting out of each side of the center of the map really put a damper on any plan I can come up with. Skills seem to be a bit short for building up to the height of the exit unless I'm using too many to contain the Lemmings over on the right.
Edit - Scratch that last one for anyone who saw it, I saw how to modify my solution for Sunsoft 12 and I've now passed it.