Seeing as this is an endgame achievement, there will obviously be SPOILERS below, so tread carefully. :)
The other guide here is very good, but I want to point out another method to finishing the TK battle.
To trigger overtime mode, you must give TK Zombrex. If you're like me, you may not realize you have to do this BEFORE the battle with Sullivan as it's greyed out as if to indicate it's time sensitive. It isn't. Just give the Zombrex to TK after he gets bitten. I don't know if you can give it to him at any point after that, but I did so before the Sullivan fight, so I know that works. TK is located in the safe house. If you go up the stairs to the cafeteria and are looking at it, TK will be to your left in a room on the left hand side of the hall.
After defeating Sullivan, you'll receive a series of fetch quest missions from TK similar to the item gathering you had to do in the OT mode of the first Dead Rising. You should have plenty of time, so just hit up the items in order. Be aware of saving often. Though you should be used to that by this point in the game, it's often easy to forget to do. There's a save point conveniently located near the Mobile Headset I believe so take advantage of that if you need to.
Once you collect everything, TK will summon you to the arena for the final battle. I recommend utilizing the save in the casino (I believe it the Americana) hallway which leads to the arena so you're as close as possible if you have to retrigger the battle.
Now for the big difference in strategy. Don't worry about prepping for the battle as you'll have all your items removed. Once it starts, go to the left and sort of behind you and run around the outer perimeter there. You'll pass and should pick up a large wrench and a bottle of OJ. Just make sure to keep moving as standing still means the pyrotechnics will hit you.
When you return to the center area where TK is, keep some distance and he'll Bum Rush you. Though you'll be inclined to dodge, DON'T. He'll charge you into the wall which will cause 3 quick time prompts: Jiggle the LS, Button, Button. Do this successfully and Chuck will push off TK and leave him stunned. Use this opportunity to whack TK 2 or 3 times with the wrench. He'll then jump out of the field of play.
Take this time to winch up Katey and Stacey. The bum rush move does very little damage, which is why I recommend this method. It's easier to take the small hit and open him up for big damage than to risk bigger damage by stepping up to him.
Don't use the winch too long or TK will hit you from behind while you're prone. Just cancel out (they drop slowly) and TK will reemerge from behind you to your right. Again, keep some distance, let him bum rush you and repeat the process. I managed to get through him without even using the OJ by conceding the smaller hits where the other method had me dying often (not that the method is bad at all, I'm just really bad at getting the roll to trigger reliably.) Also, this method is a good bet if you haven't unlocked the roll yet.
Good luck! Any questions, let me know in the comments. Enjoy the end of the game. :)