Any enemies completely immune to one of more damage type. I'm looking at you red fly and weird host hat thing...
I haven't gotten past mines, so if there's more I wouldn't know.
As for what to do about these enemies.
Red Fly should get tired after flying over a single tower or two, giving a brief window that the towers behind can hit him a couple of times.
And Host Hat... thing should have the hat as an armor rather then a lobbing damage immunity, IMO.
Especially since these two enemies appear on the same levels and are immune against what the other is weak against.