Chief is just as experienced as any of those. Since he was six years old he's done nothing but fight. He has more combat experience than a clone that experiences childhood at an accelerated rate, or a Jedi who is a keeper of the peace and has seen very little battle. If Chief hadn't relieved the Librarian's gift, he would have been composed. Without Cortana to distract the Didact, he would have been torn to pieces. I guess you didn't watch the cutscenes as well as you thought. Also, saying "anyone as capable as the chief" doesn't include a lot of people. Maybe one or two clones, the strongest Jedi and Sith, a few bounty hunters.