Half-Life 2: Episode 1
The One Free Bullet -
Beat Episode One firing exactly one bullet. Grenade, crowbar, rocket, and Gravgun kills are okay! - 40G
This was much easier to unlock than I thought it would be. It got a bit annoying when you had quite a few enemies swarming you, especially the combine. The zombies were fine. I mainly just used the Gravity Gun, Rocket Launcher and Grenades to get my kills.