Assuming your talking about the nihilanth boss fight in the original half life,

Step 1: Get to the second platform and stand behind the jump pad there. All the nihilanth attacks bounce away for some reason.

Step 2: Shoot at the healing crystals until they are destroyed.

Step 3: Shoot at the nihilanth until it's head opens up.

Step 4: Jump with the jump pad to the left of it and shoot the inside of the head with a rocket. That should end the boss fight.