I have all light world achievements (expect Girl Boy.)
I have 4 Dark world achievements, Squirrel boy, Blood clot boy, Missile boy and Dr.Fetus Boy. I am not even trying to get Zombie boy and Impossible boy. I will try Girl boy and Demon Boy. Girl boy is gonna be hard.
I think Cotton Alley is hardest just because you can't use any other character than Bandage Girl.