For this achievement it can be done in coop with the Bandana skull on for infinite ammo.
When you are back outside at the end of the level, after taking the lift up. You can just stay there and continuously kill the enemies that come towards you with the shotgun. They never run out since they continuously respawn, so happy hunting.
There is also a health pack there if the enemies damage you.
The shotgun is found on 2 enemies in the level as you are racing out and 1 is laying on a platform covered with dead enemies.
When you are racing out for the achievement which needs you to escape in under 21 minutes, when you do escape you will be in the area I am talking of. So you can easily get both achievements.
I did this and the 21 minute achievement on legendary in a single playthrough.