“The biggest pyro you’ve ever seen, the Omega Force Heavy Flamer wants to watch the world burn while toasting marshmallows over its corpse. When one steps into a battle, no one’s really safe. If you want to avoid getting barbecued, shoot the fuel tanks on his back and watch him get cooked.”
The Omega Force Heavy Flamer is an enemy you encounter after the start of the game, much like the Heavy Gunner they have thick armour and powerful weaponry. However, they are not as heavily armored as the Heavy Gunners, being able to survive only about 1/3rd as much damage (although it still takes a few dozen bullets to kill one). Also, the fuel tanks on their backs are vulnerable to damage, and can be detonated with a few shots, resulting in an instant kill.
A good tactic is to throw a grenade just behind them, the explosion will sometimes be enough to set them on fire along with any nearby enemies. From a distance they are not much of a threat since their weaponry has extremely limited range and they are very slow. But up close or indoors they are a huge threat and you will want to take them out first.