Mission 11: Atlas Falls (Veteran Difficulty)
During the mission, you will come to a room that you must breach, and your ally will push you away from the explosion. This is the room where the achievement takes place. Make sure that you keep all your grenades and flash bangs for this section, and I recommend using any weapon with a red dot sight. Once you get up from the explosion, cook a flash bang and throw it into the middle of the room. When it goes off sprint behind the counter directly in front of you. If you're fast enough, you can get away with firing some bullets and killing 1-2 enemies before going prone behind the counter. Once prone, throw another flashbang into the middle of the room. You should be able to go crouch and see where the enemies are. Throw a grenade into each corner of the room and that should take out most of the enemies. Now you can try and wait until you allies take out a few more enemies before slowing poking your head out an trying to kill any remaining enemies in the back. If you "push up" but stay behind cover, your allies will also move up and hopefully clear out the enemies. If at any point you get hit, which is indicated by a red blink on your screen, press pause and restart the checkpoint.
The achievement will unlock when the final enemy is killed and the ally says "All Clear!"
Video below demonstrates best method: