What do you think of putting a Storm card into the deck? You could storm a
Brain Freeze and mill every player without leaving blue. I realize you want the spell fist to be the finisher, but this would finish every player at once, and since it's an infinite storm, counters don't matter unless it's "counter all spells played on the stack" or something (of which I think there is only 1 version of in the whole game), or one of the 2 "end the turn right now" cards. Alternatively you could splash red for infinite
grapeshot damage, but I don't see why splashing would be necessary given a "win the game" card already existing in blue.