Go to Chapter 7 - Scrapyard Shortcut: Blind Yards. Immediately when you start the level, you'll see an STA-14 on a weapon rack. Grab it.
Fight past the enemies in this first area and you'll see a cutscene, then move into the battle with the Heavy. Shoot him in the head 2 or 3 times and he'll turn around, then shoot the glowing red tank section on his back. Lather, rinse, repeat until he explodes.