1. Combine Soldiers
Prioritize headshots with the pistol or AR2. Use grenades or SMG grenades to flush them from cover. The Gravity Gun can disarm them by knocking their weapons away or launching explosive objects at them.
2. Zombies & Fast Zombies
The Crowbar is highly effective against regular Zombies to conserve ammo. For Fast Zombies, a shotgun blast to the chest or a well-aimed pistol headshot is usually enough. Use environmental traps (sawblades, fire) in Ravenholm.
3. Poison Zombies
These carry multiple Headcrabs. A single shotgun blast or explosive can detonate the Headcrabs, killing the Zombie and scattering the Headcrabs. Be ready to deal with the released Headcrabs.
4. Barnacles
Shoot their tongue when it's extended, or shoot the creature itself. If you're caught, quickly shoot it to escape. Use the Gravity Gun to feed them explosive barrels or other objects for an easy kill.
5. Snipers
Locate their laser sight, then use the Crossbow for a one-shot kill, or the AR2's energy ball. If you have the dune buggy, you can often rush them down. Use cover to approach.
6. Manhacks
These flying drones are annoying. Use the Crowbar or Gravity Gun to swat them out of the air. A single pistol shot will also destroy them. They are vulnerable to splash damage from grenades.