Go to the point where you see a guard shooting at a monster,then kill him with the crobar for extra ammo.

When fightinhg the mutant worm thing use the aslt. rifle on its eyes then take It out with the rocket launcher.

I'm not shure if the worms where in half life maybe doom or something?

Hint: Defeating the planes.To defeat the planes that you fight on the surface you need a machine gun {kill enemy soldier}note infinite ammo helps. First kill all the soldiers then the plane come down to drop more this is your chance start shooting grenades and they will go down.