featherxquill

See, the problem with this is that it's not a fair fight. McGonagall wins because she's got magic - she can take one look at M's gun, conjure a bullet-repelling field, and then take her out with a wave of her wand. If M had magic, it'd be a tough call; if McGonagall could only use human weapons, M would obviously win.

Clearly, the answer is - they team up and rule the universe together.