tough amateur

a dilettante who will resort to intimidation tactics and physical violence if reminded that he or she possesses only superficial interest/ability in some branch of knowledge or skill

Guy 1: Don't invite Barney to help us build the deck. He doesn't know squat about basic carpentry.

Guy 2: You're better off keeping your thoughts on the matter to yourself. Barney would beat the daylights out of you if he heard you say that. He is one tough amateur!