Go into the campaign with two or more players (make sure you both do not die at once - see below).

When one of you runs out of ammo, is too hurt, or need more grenades, you can either;

A: Throw a grenade at your feet and suicide.

or

B: If you don't have any grenades, just run into the enemy and die from them.

This will give you full health, full ammo, and 2 frags. But it will make you look like a noob.

Just make sure that the player that didn't die is sheltered somewhere. This is how I beat the campaign in Legendary.