You must first finish all mission objectives in the single player game before the cheats become available. Then, during gameplay, press "back". After that, ress the following buttons to activate the cheat:

High Pitched Voices:X,A,Y,B,X
Big Heads: A,X,B,Y,A
Chicken Explosives: X,X,Y,A,B
Big Bodies: B,B,Y,X,X,A
Slow Mo: Y,Y,B,X,A
Paper Mode: B,A,X,Y,A

Every time you kill a group of enemies, save the game. When you get killed, load the saved game and kill the person that got you.

J. Stone
Finish Special Objective in Campaign Mission 4 on any difficulty

S. Grey
Finish Special Objective in Campaign Mission 5 on any difficulty

B. Gordon
Finish Special Objective in Campaign Mission 6 on any difficulty

W. Jacobs
Finish Special Objective in Campaign Mission 1 on any difficulty

H. Ramirez
Finish Special Objective in Campaign Mission 2 on any difficulty

K. Henkel
Finish Special Objective in Campaign Mission 3 on any difficulty

A. Galinsky
Finish Special Objective in Campaign Mission 1 on Recruit during a Quick Mission in Mission Mode

N. Tunny
Finish Special Objective in Campaign Mission 3 on Elite during a Quick Mission in Mission Mode

L. Cohen
Finish Special Objective in Campaign Mission 4 on Recruit during a Quick Mission in Mission Mode

D. Munz
Finish Special Objective in Campaign Mission 2 on Veteran during a Quick Mission in Mission Mode

S. Ibrahim
Finish Special Objective in Campaign Mission 6 on Elite during a Quick Mission in Mission Mode

G. Osadze
Finish Special Objective in Campaign Mission 5 on Veteran during a Quick Mission in Mission Mode

Start a private game in cooperative mode. Play the game on the elite difficulty setting with unlimited respawns and grenades. You will get a win and 45 kills easily.