( Must be on Co-op to do invincibility.) At the start of the level buy a new gun and then go to the corner where outside there is a truck. You or your partner must prone in order for the other person to jump on top of you, then u stand up and BAMM u r invincible. Just don't move from that spot and you can't be killed. Also a hint don't try to knife far away zombies because you'll pull away from ur partner and most likely die. Also another hint is to have a flamethrower so u dont run out of ammo. Good luck!!!