The Orange Box

Found an even easier way to get this, you can end up getting it before you even get to the lift. I'll start out by saying it must be in one play through. Another tip is to quickly backpeddle or crouch when you set a zombie on fire so you can carry on using the flare. Using the invincibility cheat makes this a lot easier. It's LB, Up, RB, Up, LB, LB, Up, RB, RB, Up. Nothing will happen so you'll have to hope it worked.

Load up the level "Lowlife" on Episode One and When you get the to first area with all the zombies sprint past them and go into the door which leads to the locked door, open up the vent as fast as you can and when you look across to a dead body there's a flare there, save here and then pick it up and go back out the way you came in and there should be 2-3 zombies coming through the corridor, crouch and try to set at least 2 of them on fire.

Now in the next area where there's a metal cage where you grab your shotgun and pistol there's another flare in there, turn the power on and save here, take the flare to the door with the car in the way, drop it, hit the car with the Grav Gun pick the flare back up and get 3 of the 4 zombies outside of here. NOTE: One of them is lying down in the corner and looks dead but isn't. You should now have 5/15.

After the combine zombie or "Zombine" kills himself save here and then run outside towards a door with a fuel canister next to it, next to that is another flare, try and get as many of these zombines as you can and if the flare gets hit pick it up ASAP and get flaring some more zombies. Watch out as zombines are pretty suicidal.

In the next place where you got to turn the power on there's another flare. Save here, Turn it on and pick up the flare, try to get outside because if you get blocked in it's a lot harder and then try to get as many of these zombies as you can, you should at least get 2 of these.

Keep playing until you get past the area where you plug the antlion holes with cars, you should be now in an area with ankle high water, save here, run past all of the zombies and go upstairs, to your right there's a flare, grab it and rush back downstairs and get as many as you can. I got my achievement here but if you need 1/2 more keep playing until the elevator/lift section.

For the elevator/lift section just memorize the flare spawns and save before you kill anything. If you haven't got it by now just reload an old save and keep trying.

All the solutions so far are great but reading it is not as good as watching it.

English Gamerrr brought up a good point trying to get at least 2 zombies in the first section by sprinting past all of them grabbing the flare in the vent and quickly burn the 2 zombies that come through the door.

If you set the first 2 on fire in the beginning and follow the video you should have set 12 zombies on fire by the time you get to the elevator section. Getting 3 more is a piece of cake in the elevator section because there are numerous flares lying around.

(I take no credit for the video)