Next I went to clean up the last 7 red skulls that I'm missing... and that involved having to beat the bosses again in each level where I was missing some.
Iron Whale is my least favorite level... water physics just aren't fun in this game.
That one part in Explodatorium still gave me some trouble.
That tower minigame... it's so ridiculously hard, I still can't get much farther than like half way. Even the smallest mistakes add up, and you can only afford so many mistakes before the electricity catches up.