I was pretty surprised there were no guides for this, admittedly I had to click around quite a bit to find what actually popped this.
From the Main Menu:1.) Go to
Multiplayer > Custom Game2.) Scroll down to click
Game Mode and select
343 Industries3.) Press
on any Game Mode to
Save As a new mode
4.) Add (or remove) a character and press
This should save a copy of the game mode as your own and pop the cheevo.