Why do you think Trump could defeat her. He seems to have an even worse case of foot in mouth disease than Tod Akin and I think he would give Hillary all the ammunition she needs to destroy him. On the other hand, I think that Ted Cruz or Governor Walker are more conservative and also much less likely to say something stupid and self destruct their campaigns.
If someone else wants to run against Hillary, then they better get started. So far Trump's the only one who's getting any sort of message out there.
Of course the left and the GOPe doesn't like him... they won't like anyone who might actually threaten Hillary.