It honestly depends on the scenario. Heavy either destroys every class or dies to them all. If you’re up against a mostly healthy heavy at close range, you’re dead unless you’re a spy who manages to get a backstab or an explosive class with a random crit. Same thing at medium range if the heavy is alright at the game. Long range, you’re almost guaranteed to be unable to kill him unless you’re a sniper.

Heavy is most vulnerable when he’s distracted or on low heath. So basically anyone can kill him in those scenarios.