Here is the full list of all 13 Half-Life 2: Episode One achievements. It takes around 3-4 hours to unlock all of the achievements on Windows.
Survive long enough to get on the parking garage elevator.
Contain the Citadel core.
Make it to the bottom of the Citadel's main elevator shaft in one piece.
Contain the Citadel core without killing any stalkers.
Use the cars to squash 15 antlions in Episode One.
Help Alyx snipe 30 enemies in Episode One.
Destroy the gunship in the hospital attic.
Don't let any citizens die when escorting them to the escape train.
Escape City 17 with Alyx.
Finish the game firing exactly one bullet. Grenade, crowbar, rocket, and Gravity Gun kills are okay!
Kill five enemies with the same energy ball.
Kill an Elite Soldier with his own energy ball.
Use flares to light 15 zombies on fire.