Pyromaniac Achievement in Mass Effect 3
Set 100 enemies on fire with powers.
How to unlock Pyromaniac
There are a few powers that result in the enemy becoming on fire. These include Incendiary based powers. Incendiary ammo with the Soldier class is going to be the easiest.
Even after dying, the ones you have obtained count. They count across all playthroughs as well as multiplayer (which you can play by yourself). Must be done by your Shep! Squad uses do not count.