Jeb may have been an easy target, but Rubio and Cruz were not slouches. Especially Cruz, who's a well known master debater (hahahahah... not actually, he's really good). Annhilated him with that 9/11 riposte. So I wouldn't underestimate Trump. He's dangerous, he made it through the cesspool that is he republican primaries with multiple debates and a 10 person shit show.
This guy is good or just a very very lucky mofo, but ignore him at your peril. He's got some skill even if his ideas are wrong, or racist or not well thought out. He's a good public speaker who knows his audience, and their language level.