Left 4 Dead

Molotovs or gas cans are required for this. There are two basic ways to pull this off: One is to simply lob the molotov at the witch. This is easiest to do when the witch is on the ground or in an area that allows you to at least have it land close enough to spread to her. Obviously the molotov works best here, but if you can manage to get a gas can near her without startling her first, that works well too. Once the gas can is placed, shot it and be ready for a very upset yet crispy lady to come visit you.

The second is to induce her to run through the flame. I found this to be effective when the witch is on a second tier or small platform like the top of a bus. In those situations, getting the molotov to land near her is difficult since it may land below her affording no damage (and thus no flame) what so ever. In those cases, take a look around and determine the most likely path the witch will take to get to you. In fact, if you can leverage yourself in a hall way or other choke point, that's even better. The ideal point is one that still allows you a shot at the witch.

At that point, the setup is fairly straight forward: get into your choke point and light a fire in front of you (well in front of you in the case of the gas can). Once the fire is lit, shoot (or otherwise disturb) the witch. She'll run at you and thus through the wall of flame.

Note that in either case, she will still get to you. She'll just be on fire with a little less hit points than she had when she started. Be ready to deal with her.

Step 1.Grab a molotov.
Step 2.Find a Witch.
Step 3. Aim your molotov at the cry baby.
Step 4. Throw the molotov.
Step 5. Run AND shoot!

Burn the Witch - 10G (Original Achievement)

Difficulty: 2/10

Throwing a Molotov at the witch is the obvious strategy here, but as said above you can hide in a corner and throw the Molotov in front of you. Afterwards just shoot the witch and she'll come running through the fire to get to you and there you have it. You can also simply throw the Molotov at the witch but some people aren't as accurate as others.

For those that don't know, a Molotov is a bottle that bursts into flames when thrown. They are normally located in a bundle or sometimes in random locations.