-( Story related, cannot be missed )-
After destroying the Maimtron 9000's power source, you'll find its remains lying outside the front of the office. Jimmy has reconditioned it to be a boxing arena and will take on any challengers, though with his boxing glove he is far too cocky to ever see the Horseman of War given up. Using the Boxing Betty that you can obtain from Santa's Workshop, attempt to enter the ring.
The first two combatants will pose little challenge. If they're readying on the right, they'll be striking to the left, meaning you will want to feign away (pressing the directions on the control stick to move away) and then go in for the strike (using Square for left and Circle for right). The only one that might pose a problem would be Jimmy himself.
For me, the game had some issues with actually showing tells for him. To fix this, attempt to punch him once or twice, which he will guard through, and then he will show you the tells. Be careful feigning right if you don't see any tells, as one swift punch from the boxing glove will immediately take you out. Thankfully, the game is forgiving and you will resume back at the final match. Just eliminate Jimmy as best as you can to claim the spoils of War.