At the main menu, press R(2), L(2), B(2), Y(2), X(2), White to unlock the cheats menu under the "Options" screen.
At the beginning of the level, get all the ammunition in the weapons room (to your left at the very start). Then, move on to the Major. When he tells you to go to the bunkers, take Ragman and the Doc and crouch down next to the Major. Then, take Junior and put him in the first bunker to your right and up the stairs. Then, take Hoss and put him in the next bunker down. Whenever someone dies, the Doc will heal them.