This glitch is very easy to do and im currently over level 600 on pc. Step 1: Tell your friend to go into a zombies custom match and wait. (make sure he set the difficulty to realistic) Step 2: Go to multiplayer then custom game. Step 3: press "O" or whatever your social button is linked to and then click the friends tab. Step 4: use your arrow keys to navigate to your friend that is in a zombie lobby and spam space till you are in his lobby with the menu glitched on(this is important to use your keyboard and not your mouse) Step 5: Press "O" or "F3" then tell your friend to back out and bring you with him Step 6: Press "O" or "F3" then tell your friend to go into a private match and leave without you. Step 7: Edit the settings however you like then press escape and start the match. Some things I would do for maximum xp is make sure there are 3 bots in the game and you set friendly fire on then team kill them. (set a max number of downs so they dont bother you) Also set vermin to only that way it will be only dogs. The best way to earn xp while doing this custom mutations glitch is to do the unlimited ult glitch and use either the death machine or the ragnarok thing. In order to do this you must get a thundergat or w/e its called then pap your other weapon leave it in there then turn ur thundergat into the fireplace and leave it there then use your ult it should now be unlimited. Now with your unlimited ult find a glitch spot and sit there till your game crashs at first i was doing the docks spot but ive found that the lightpole spot on the roof is much better. In order to get in the spot you must have the ragnarok special and basically all you do is jump and then at the highest point of your jump use your ability. With the death machine you dont need a glitch spot u just need to find a corner and have sometihng hold the trigger down for u and it will be unlimited. My friend sits in the corner by the pap on the roof. Hope this post was atleast a little helpful.