At the mission selection screen or pause menu, hold LT + RT + Back, then enter the following code by pressing the according buttons.

Code - Effect

100% Health - LB(2), RB, X, RB, Y
Infinite Ammunition - RB(2), LB, X, LB, Y
Team Invincible - X(2), Y, RB, Y, LB
Scott Mitchell Invincible - Y(2), X, RB, X, LB
Unlock All Missions - Y, RB, Y, RB, X