Credit to xxpokoxx

Not my text tutorial i just want the glitch out there

STEPS:
1) With high level account, go to xbox live
2) With the lower level account, sign in
3) With the lower level account, sign out completely
4) With the player 2 controller, press A to sign in
5) With the higher level account, press B to sign out
6) With the player 2 controller, sign into the higher level account
7) With the player 1 controller, sign into the lower level account
8) The game will bring you back to the Multiplayer menu
9) With the higher level account (Player 2 controller), go to xbox live
10) With the lower level account (Player 1 controller), press A
11) With the lower level account, sign out completely
12) With the lower level account, press A to sign back in
13) With the higher level account, (Player 2 controller) press B to sign out
14)Now you want to switch the higher level account back to the player 1 controller and the lower level account to the player 2 controller
15) Press Start and the lower level account (Player 2 controller) should glitch out and be the same level as the higher level account
16) With the glitching account (Player 2 controller), go back to the Multiplayer Menu, then back to Campaign
17) With the glitched account, press Start and go to Xbox Live
18) You will now see that the old lower level account is now the same level as the higher level account
19) Enjoy!

Also you get the other players stats and custom classes also this is permanent

Video link
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Credit to xxpokoxx on TTG