The Badass Pryotech Heavy is a tougher version of a normal Pyrotech Heavy.
The pack on the Heavy's back is his weak point. Well placed shots will not only stun the Pyrotech for a couple of seconds, but when damaged enough will cause them to explode. This explosion can inflict considerable damage to any nearby enemies caught by the blast and leave behind a lingering pool of fire for a few moments.
Unlike a common Pyrotech Heavy, the Badass Pyrotech Heavy does not waddle and will break out into a jog towards the character. While it is still significantly slower than the character, its increased running speed will be coupled with the booster jets normal heavies use to move around, and it will more often create numerous simultaneous molten puddles. It will almost always be in the midst of the character's attempts to move around, and can be difficult to avoid if a character does not quickly leave the area that it is in.