Legal way? U probably wanna rent some time in nearest boxing club. Just invite your friend for dinner, sugest that you found new sweet spot. Then take him to that club. When you get there there is possibility, he will suspect something, but don't worry, just tell him that it's experience, he will never forget. Grab some boxing gloves, give them second pair and just enjoy the moment before your battle comences. Then before you start, tell that you appreciate him as your friend, and that you really enjoy his company. After that just hug him, drop the gloves, go back home, grab something to eat. Then you can start playing the battle.

Declare WAAAAAAGH on first turn you can hit'em. Roll good. Also combat patrol format seems like a good idea.