I can't beat the Nihilanth

[pasta864]

He only opens his head when I die. Is that normal?

[inkus]

AFAIR, it's not necessary to wait for Nihilanth to open his head if you do concussion damage with rocket launcher for example.

[Cat]

Shoot its brain.

[♠Gamer3106♠]

His head doesn't need to be open

Just destroy the crystals and blast him right away.

[Snark]

Have you tried talking it out?

[Sunk Cost Fallacy]

Step 1: Destroy the 3 yellow crystals around him first, otherwise he will keep healing/regen his HP

Step 2: Shoot him until all the orange orbs is gone, THEN he will open his head

Step 3: Run to jump pad and shoot inside his opened head

Step 4: Done

If he closes his head again, don't worry, just keep shooting him and he will open it eventually, then repeat Step 3.

[Ffabbia]

Once his head is open, machine gun grenades are the most effective tool, in my opinion.

If you're really struggling it also makes life considerably easier if you go through all the teleporters he summons. It makes the fight slow, and some of the rooms are annoying - BUT - it's more Half Life!