You will have to repair the elevator at some point (by shooting the rockets under it to bring it in position). After you did that return to the room with the bench and save.
A good way to do it is by using the fully upgraded Detonator. (You can save in the round room with the bench in it and sell all your gear/rearrange your power nodes to upgrade the detonator. After you're done just load the last save.)
Stand in the center of the round elevator and don't move. Don't aim yet. Use the left stick to look around and wait for the first tripod to appear. Then aim your gun and shoot one grenade at the center of the tripod. Stop aiming again and repeat until the achievement pops.
Isaac will aim the way you point the camera immediately, even if he is facing the opposite direction. This is much quicker than actually turning him around.