This achievement can be completed as a Pyro, on any map and gamemode.
First, your team need to have at least pyro.
Second, let your pyro teammate to ignite an enemy player.
Third, just finish him.
After that, the achievement is yours.
The problems, it's kind of rare, so it's about to play a lot of pyro.
The achievement will be completed automatically after finishing the enemy, ignited by your teammate.