This custom mutations glitch is confirmed working, I'm in a lobby right now. I will also include tips to get the most out of this glitch.
P.S - This glitch is much easier to hit if you have two Xbox's. It's almost impossible if you don't do it that way.
Setup (Do this first)
1. 1st Player needs to go into Multiplayer -> Custom Games and use D-Pad to hover over the options button. Then he/she needs to open social and hover over Player 2's name.
2. 2nd Player needs to sit in a custom mutations in zombies.
How to do the custom mutations glitch
Step 1: Player 1 needs to spam A/X on Player 2.
Step 2: This is really tricky for the timing, Player 2 has to back out and bring party the split second Player 1 joins the game, how you time this, that's up to you.
Step 3: Player 1 should still have the stack, proceed to continue to a private match and leave alone. If you got the timing right, Player 1 will be able to edit the settings.
TIP: Utilize this glitch for XP
The best thing you can do if you are trying to level up quick is configure the game:
1. Turn all zombies off, set vermin to only. Set spawn rate to quadruple.
2. Set zombie move speed to sprint for the minimum setting, and super sprint for max.
3. Give yourself whatever else you'd think help, just don't change the zombies settings.
4. if you feel comfortable with it, set the difficulty on realistic, it will give you loads more XP.
At this rate, you will prestige 1-55 within 2 hours tops. That's 2 hours per prestige. Good luck!