I know it's a dumb question, but how would I gather a list of mobs in a named Z level?
for(var/mob/enemies/E in ?)

What would I put in place of the question mark to refer to only a certain Z level without any reference to a mob or obj?
You can use the block() proc to generate a list of turfs.
in block(locate(1,1,1),locate(10,10,1))

That assumes your map is 10 by 10, though.