I can't think of any other hack in the KHUT that let me do this besides the Fail Mission EXP x120 hack so that's my basis on why you can still gain multiple levels from using this hack by COMPLETING the mission.

[Warning] Also I don't know if this can cause a fast/instant MP-Ban (Multiplayer Ban)[Warning]

Steps:
1. Enable EXP-x120 hack before pressing the play button at the main menu
2. Pick your character, load up a mission (I just do Onslaught), and kill some zombies for EXP. A couple will do it, you don't gotta wait till the last couple seconds for this.
3. Quit the mission and proceed as normal
4. Reload the map/mission or a new one.
5. Play it as normal.

Presto~ This can be amplified with Increased Spawns Hack but DON'T pick maps that force you to clear out an area unless you're ready to be playing for a while. I spent over an hour on Survivors which in essence wasn't worth it. Time/Reward people.

Overall, I got a character to jump levels 1>4>8>41>46. I wasn't consistent in map picks/zombie killing so that's why the level jumps are varied.

EDIT:
So it's been about 12 hours since boosting a new account with this method and I haven't been MP banned. The only real problem is that people will probably find you suspicious with no/very low MP stats.

Great post. I paired this with the Legit MP Stats hack at the final of my level, My biggest issue was getting deaths.

Instead of quitting, I like to put on Glass Cannon, Light up some zombies and let them one-tap me, that way I get the Death count... But come to think about it, I'm not sure if that counts as a "Multiplayer" stat or not.

Increased Zombie Spawn on Zombie Pods made this method extremely quick.

Trying to find a way to customize the Legit MP Stats myself, to tune it to my level, if I don't want to climb all the way to level 50. level 35 is a pretty good spot, though many people back on you when they see your level. I might try for Level 50 right now, just for the heck of it.

Cool tips!