It always feels cheap how you can jump on them the moment they come out of their shell spin, resulting in an easy chain victory. I'm hoping that in a future Mario game (it's tricky to do this in SMM2) that they will be given a brief invulnerability period after doing their shell spin. If they have this, attempting a chain-stomping will get you hit, so you'll have to wait until after they attack again before you can hit them.

This could always be added if said Mario game has some sort of difficulty setting.