Wait for slow motion to get up to full and have a shotgun. When ready, activate slow motion and fire as many rounds into the armor as you can. During this, it is also a good idea to circle around it to throw off its fire. This tactic is not useful if slow motion is off. The Particle weapon, Repeating cannon, and Rocket Launcher also does significant damage against them. If done correctly, Slide Kicks will kill a Heavy Armor in one hit.
Use the semi-auto rifle or plasma rifle. Zoom in and shoot it in the head, then in the upper legs where there is no armor. The Heavy Armor soldier will double up each time and cease shooting. Repeat your shots from head to upper legs until it is dead.