This achievement is very easy to get. All you have to do is Kill a Mummy by setting it on fire. You can do this with a fire weapon or the environment like an oil pot. The easiest way I found was using firebombs.

You can find mummies in the Aaru, the first afterlife you visit. Go to a location called "Dock of the Eclipse" here:

You will see a few Anubis guards and one enemy that looks like this:
Now just kill him with firebombs or whatever fire weapons you want, and you should unlock the achievement.

That's it! Hope this guide helped :)