Invulnerability Hunter

The Invulnerability Hunter is a boss monster found in Doom 3: Resurrection of Evil, the third and final Hunter the player will encounter. Similar to the Berserk Hunter, the Invulnerability Hunter is noticeably larger than a Hell knight, and it has an extremely muscular physique with a blue electric aura surrounding it. This aura will intensify when the creature becomes invulnerable by absorbing electricity from the nearby power coils, giving off lightning bolts. It occupies the Energy Core area of Phobos Labs - Sector 2: Molecular Research for this purpose.

Tactical analysis
The Invulnerability Hunter starts off by drawing power from the four power coils in the room, but one immediately overloads and is destroyed, interrupting its attempt to become invulnerable - this is a clue about how to defeat the monster. It will be open to damage for a short period, during which it is best to utilize the power of the Artifact to maximize damage dealt. After this, it will absorb power from the remaining three active power coils in the room, rendering it invincible. Destroying the open coil will again cause a temporary shutdown, leaving the Hunter vulnerable. When the remaining coils reactivate, repeat the process. When all the coils are destroyed, the Hunter will no longer be able to retain its invulnerability. One can hide from the Hunter by running underground, but it will stomp the ground to damage the player should he be standing up. Note that this underground area was removed from the version of Resurrection of Evil that is included with Doom 3: BFG Edition.

When it is vulnerable to attack, use the Artifact's helltime ability, pull out the chaingun, and start firing until it becomes invulnerable.

The Invulnerability Hunter's attacks include hurling an electrified plasma orb and stomping the ground to unleash an electric shockwave. This wave cannot hit the player if he is at a higher elevation, so there are safe areas in the room that can be used to escape it.

While it may be tempting to try to use the Berserk power of the Artifact to conserve ammo by punching the Hunter with the player's fists, one will find that the electric properties of the monster render it invulnerable to melee attacks - instead, damage will be dealt to the player and the Invulnerability Hunter remains unharmed. The double barrel shotgun, chaingun, plasma gun, and rocket launcher remain the best choice of weapons due to this fact.

Killing it will gain the invulnerability power for the Artifact.

Trivia
One of the textures used with alpha blending to create the electric effect around the Invulnerability Hunter is present in the base files for Doom 3. This was initially thought to be an unused monster by fans on the Doomworld forums until it was properly identified.