This should be acquired with the Hard to Kill achievement.
When playing co-op, you want to proceed slowly as there are many enemies. So many, in fact, that you will definitely make use of any ammo you find. Almost every time you stand at a corpse, you will get bombarded by enemies from all sides.
NOTE: IN CO-OP, YOU CANNOT ROTATE THE CAMERA. It is fixed at an angular position, thus making it a little more challenging to see your enemies on the screen at all times. Make sure you split the coverage load between your partner (one player cover one direction, the other player cover the other direction), and maintain communication regarding ammo and health. Whoever is in more need of whichever should be the one collecting and checking corpses. More healthpacks are found in corpses and more ammo is found loose (however there still is ammo on corpses).
If you haven't used grenades in the campaign, you should make use of them here. They really do come in handy.
Just make sure to cover each other, and don't let your health go below yellow.
You can also use doorways to force enemies to narrow up their path of attack.
Other things to note about the level: It's medium-sized, and doesn't require much back-and-forth between objectives. Take a moment to deviate from your waypoints to collect ammo and health off lockers, corpses, or the ground, and always expect an ambush at each of these locations. The weapons available in this level are: Pistol, Assault Rifle, Shot-Cannon, Laser Rifle, and Ionspike. Make use of the stronger weapons too, and don't try to conserve too much ammo at the sacrifice of health. Health should be the main priority (especially if you're going for Hard to Kill).
Be patient, play slowly, and you should get this achievement with Hard to Kill (if either of you don't die) in about 30 minutes.