What Mitchell did wrong was he misjudged his own early work.
Mitchell basically threw caution to the wind because he thought he had Banks out of there.
Banks caught Seth with a shot that visibly shook him, and didn't let Mitchell off the hook.
But Mitchell's only chance against either Kiltschko was catching them with something big early on. I think if they could get Seth to the 5th or 6th round, he would have gassed and been easy pickens.