Chapter 7 “Scrapyard Shortcut” – Blind Yards, immediately after the area as described in Power Spike trophy above, you’ll come to a large yellow structure on your right and an ammo box beside a weapon rack containing the StA-14. Grab this while you can because shortly after you move forward a cutscene dialogue between Rico and Jammer will play, followed by your first encounter with a Heavy.
The Heavy is a heavily armored Helghast with a chaingun. The weapon rack is blocked after the cutscene so you’ll have to restart the section if you didn’t grab the gun prior to the cutscene. Aim for the Heavy’s head a few times to make him turn around, and quickly shoot the horizontal red canisters behind his back. Repeat this a couple of times and you’ll take him down.