Half-Life Source is such a broken garbage that you can actually pass the whole game without being hit by any enemy with bullet-based weapon (HECU soldiers, Human Assassins, Helicopters, etc.). All you have to do is:

1. Watch at the floor or at the ceiling (sky/whatever is above).

2. On the flat surface bullets will just fly above your hitboxes without dealing any damage. When surface is uneven, from some angles bullets may hurt you but the chance is extremely low. But don't forget, that melee kicks and explosions are still dangerous for you.

3. The End.