Select the Veteran difficulty setting and play mission 11, "Atlas Falls". Progress until reaching the room where you get pushed away by your teammate (as you try to breach in) because the enemies are already shooting through the doors. After the door breach, stay back to the left of the door. Fire at the door to hit a few enemies that pop out of there. They will have difficulty targeting you. Eventually, Merrick and Keegan will move up through the door. Hang back and do not enter the room. Toss cooked frag grenades into the room, but do not enter the doorway. Be patient and kill a few enemies, and allow Merrick and Keegan do most of the work for you in order to avoid damage.