Sign in an extra controller even if you do not have anyone else to play with in the game. For whatever reason, you end up having more overall hitpoints or at the very least this split's the enemies focus.
Whatever it is, you end up dying a lot less and this can be particularly useful when you are stuck at a checkpoint where you have very little health and/or ammo for your main weapon.
Trust me, this is VERY USEFUL
as there are plenty of spots in the game where you just keep on dying because you have no health and there are too many enemies to focus on before getting gunned down.
So what you do is sign in a guest and then move their reticle off the screen so it's out of the way and NOT distracting. You can sign in an additional controller at any point in the game without having to restart as it's drop in / drop out.
Also, if anyone dies it DOES NOT
restart at the last checkpoint, as long as one of the controller's player's is still alive. This is also helpful in getting the No Turning Back Achievement for completing level 12 on Veteran without using a Checkpoint.