If I were him...well, let me preface that by saying I wouldn't have done even a percent of the tarded crap he's done, but anyway...I'd sit back and let Fox have at her. Make my points clear and concise, don't allow myself to be provoked and spell out some serious policy issues that have been missing thus far.
The final debate is where it comes down to the undecided voters. It's been a clown show so far with no winner heads above the other in any of them. Now is the time to let Fox go after her on policy and the other items the other debates have ignored and for him to act "Presidential."