In Half-Life 2: Episode One, the last underground level is where Gordon and Alyx find an unpowered elevator, look around for a switch to power it on, and then try to survive the zombie onslaught while waiting the elevator to arrive.
In my previous playthroughs, I always struggled on this part, and could only survive after a few tries. (Damn you, massive zombie horde! Damn you, zombies carrying a grenade! Damn you, slow elevator that takes forever! Damn you, flashlight that doesn't have enough battery!)
Finish the game firing exactly one bullet. Grenade, crowbar, rocket, and Gravity Gun kills are okay!
I tried getting it, but I'm repeatedly failing in this level. If using shotgun and sub-machine gun against a massive number of zombies in the dark was quite difficult, not using these weapons seems impossible! Half of the time I die, and half of the time Alyx dies. (Before trying to get this achievement, I didn't even know that she could die!)
So... What are the suggestions to pass this map? How to survive long enough? How to get this achievement?