Obviously, you will need to obtain the dagger before you can attempt this achievement.
If you need a tip for that, check my guide for:
Once obtained, you must equip the Ceremonial Dagger to the Bright Wizard. Then start a level on Easy so you can mow enemies down - I don't think it has to be one of the DLC levels, but play one of them just to be safe.
When attacking enemies with the dagger equipped, you'll want to use the charged attack by holding the
for a few beats before letting go. Charged strikes from the dagger will hit multiple enemies and should light them all on fire, so look for groups. You do not have to kill the enemies, just light them on fire. It doesn't take long at all once you get the rhythm of the charged strike down. Normal attacks do not ignite enemies.
The achievement will unlock upon igniting your 100th enemy.