I'd say Supes turning him into ice, smashing said ice, then vaping what's left with heat vision should be enough to take Buu out. Heat vision harms guys tougher than he is sI by rights he should be able to dish out a condensed beam of energy that has the same output as a supernova based on that alone imo. So i don't see a Buu that has his healing abilities frozen solid coming back from that.