In the last level where you have to find Whitley as an objective, as you stand in the elevator strafe left and launch a grenade just outside the opening. If done correctly, you can bring Whitley with you to help kill all the troopers. Note: You have to give him some weaponry.
This Boss has a sweet spot. If you get directly under it, it will not attack. When you get under it, blast away until its health is in the red, then burn it.