Part 61

As the chapter loads, immediately hold the 'Move backward' key. Release it when your view begins to fade, then run forward to the base of the large, spiky rock directly ahead. This should hide you from Nihilanth's initial attacks. If he fires a teleporter orb, shoot down any appearing vortigaunts or controllers, or quickrestore and try hiding faster.

Locate the three glowing yellow-orange crystals high on the chamber walls. Your view of one may be obstructed. Carefully move behind the rock to see and shoot all three crystals without Nihilanth spotting you. The crossbow is recommended, taking two darts per crystal (four darts per crystal according to SRC).

With Nihilanth's healing ability disabled, begin attacking him. Carefully position yourself to shoot one side of his head without being seen. Use the gluon gun, .357 Magnum, and crossbow, but save RPG rockets for now. You can use hornets, but ammo conservation is unnecessary as this is the final battle.

Shoot Nihilanth's head until it opens. Continue shooting the exposed head while he moans or growls.

Once he stops moaning, quicksave and equip the RPG. It's possible to hit Nihilanth's exposed brain spike from cover with a rocket. Fire rockets, aiming for the approximate center of his head, until he starts spinning and spewing energy or you run out of rockets.

If you run out of rockets, quickrestore and try again. If Nihilanth starts spinning and spewing but the game doesn't transition to the 'Endgame' chapter (a common glitch), quickrestore and try again.

If the glitch persists or rockets are ineffective, use the 'whoosh-thing' on the ground to the right of your rock to fly above Nihilanth's head. Shoot his glowing brain spike with the assault rifle or another weapon. Alternatively, land on his head and attack the brain spike with the crowbar. This method should trigger the transition to the 'Endgame' chapter. (SRC: Neither the rocket strategy nor the crowbar will work, so you'll need to whoosh up and shoot his brain spike with something.)