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.
Use flares to light 15 zombies on fire.
Kill five enemies with the same energy ball.
Help Alyx snipe 30 enemies in Episode One.
Finish the game firing exactly one bullet. Grenade, crowbar, rocket, and Gravity Gun kills are okay!
Contain the Citadel core without killing any stalkers.
Kill an Elite Soldier with his own energy ball.
Escape City 17 with Alyx.
Don't let any citizens die when escorting them to the escape train.
Destroy the gunship in the hospital attic.
Contain the Citadel core.
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.