I agree with @Dark Lord Tarkas. The clones had the element of surprise - the Jedi didn't sense or suspect their betrayal (as far as we know). Cad Bane defeating Ahsoka, that's one thing. Holding his own against Obi Wan? Can't see it. Same with Grievous, I'd be alright with him taking out padawans and knights, but not a master. At least not with out help.