No missable achievements (plus 13 unknown)
Sign in with Steam or Xbox to track your progress, and:
Make it to the bottom of the Citadel's main elevator shaft in one piece.
Contain the Citadel core.
Survive long enough to get on the parking garage elevator.
Destroy the gunship in the hospital attic.
Escape City 17 with Alyx.
Don't let any citizens die when escorting them to the escape train.
Help Alyx snipe 30 enemies in Episode One.
Kill an Elite Soldier with his own energy ball.
Use the cars to squash 15 antlions in Episode One.
Contain the Citadel core without killing any stalkers.
Kill five enemies with the same energy ball.
Finish the game firing exactly one bullet. Grenade, crowbar, rocket, and Gravity Gun kills are okay!
Use flares to light 15 zombies on fire.