In order to complete this achievement you have to set 100 enemies on fire using only Shepard's powers. Companions' powers do not count towards this achievement.
Enemies can be set on fire by using next powers:
- Incinerate (Infiltrator, Engineer)
- Incendiary Ammo (Soldier, Vanguard)
- Inferno Grenade (Bonus ability from Ashley)
If you play any of the classes mentioned above, you'll naturally get this achievement during your playthrough by simply using Incinerate, Incendiary Ammo or Inferno on unshielded enemies.
If your Shepard is Adept or Sentinel and you don't want to bother with Inferno grenade or don't have Ashley in your team, you can do this:
1. Start a new game and choose an engineer
2. Get to the part where you have to repel the waves of cannibals near the gunship, waiting for Normandy to come for help (the very first mission with Anderson).
3. Don't waste too much ammo (cutscene with Normandy triggers when you're out of ammo) and just spam incinerate on cannibals until you're done.