Like the other Hunters, the Invulnerability Hunter resembles a Hell Knight with a vaguely greenish-brown color, though it is noticeably taller (roughly 10 ft.) and more muscular. Its head has two sharp points on the cheeks, seems to have no eyes, and the flesh around its nose and mouth is noticeably more hollow (giving its head a skull-like appearance). It has a faint blue aura around its body, which intensifies to almost completely envelope it when it absorbs power from the power coils.
The Invulnerability Hunter attacks by either throwing two large electric energy balls from above its head or doing an electric ground pound shockwave with a large radius. It can also do a clawing combo attack. Its main gimmick is absorbing energy from the power coils in the room, giving it a large blue aura and making it completely invulnerable to damage (hence its name).
To defeat the Invulnerability Hunter, the player must evade its attacks and do damage to it until it becomes invulnerable. Once this happens, the player can then open the cover to expose the core of the coil. The core must be destroyed to destroy the coil (this is marked by the automated message: "Core [number] Failure") one-by-one so that it cannot turn invulnerable anymore.
The electric balls can be circle-strafed and the ground pound can be avoided by staying above ground (whether by jumping or by staying above the stairs in the room). There is a med kit at the far end of the room, and a hatch that leads to a compact crawlspace with lots of bodies (to charge The Artifact), an ammo belt for the chaingun, and a health station. Beware though, the player must be crouched while in this space, less the Hunter damage the player with its shockwave.
The player has a narrow opportunity to destroy the core of the power coil before the cover comes back over it. It is recommended that they be destroyed from a distance, as when destroyed the cores will catch fire that will damage the player should they get too close (and potentially push them back towards the Hunter). The core require approximately 500 points of damage to destroy. The rocket launcher is an ideal weapon to destroy them, doing so in about 3 shots. The Artifact's Helltime power is very useful for destroying the cores and the Berserk power for attacking the Hunter.
Killing the Invulnerability Hunter will allow the Artifact to absorb its invulnerability ability, allowing the player to become immune to all damage (without the need for absorbing energy like the Hunter) with the exception of the Maledict's attacks.
Doom 3: Boss FAQ - posted by D00MBuggy.