Follow these steps in order for the glitch to work.
1.go in to the splitscreen mode, and set the game options as free for all on nuketown, and put enemies on 2.
2.back out of the split screen lobby and hover over the split screen option.
3.open your in game friends list and look for a friend who is sitting in a party.
4.click square on your friend to join his party and immediatly after press the X button as fast as you can. This should put you into a game of nuketown free for all.
5.if all has gone right your *online* classes that you use in multiplayer games should be available for selection.
6.once you have chosen your online class, the class that you appear to be holding will not be your online class, it will be the default class from the split screen mode.
7.look for an enemy and kill him. (GET ONE KILL ONLY) then quit the game (you may have trouble quittin the game at first, it wouldnt let me press the leave button on the first few attempts)
8.if all has gone correctly, when you check your combat record you should have a killstreak of 1 and a winstreak of 0.
**You dont have to get just one kill only, i thought id give it a go at trying to reset it back to my previous LEGIT highest killstreak which was 30, so i opened up the same game and got 30 kills without dieing then left the game, and my killstreak reset to 30 from 189 **