I like the egg. If you can get a setup where the enemies get in range to hatch the egg but die before changing blows with the bird or whatever it is you can place another egg directly underneath it, then hatch that one, on and on forever until you get a square packed with the things that just instantly melt anything that comes into range.

I like the firebat egg too, especially when the egg opening causes damage too. A firebat on top of another trap will usually result in monsters fighing the bat (some monsters will ignore it though). If they stopped to fight, it gives the trap time to cool-down and run again.

freeze bomb, works wonder with tesla & tomb mages