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