Earliest this can be done is 5-2 Chamber of fire
At the end of the level you get to kill an enemy with a fireball, once you go past in the next area there will be 4 enemies there, reload and repeat getting 4 each time as the single enemy in the previous area will not respawn as you've advanced into the next area
It will take 3 reloads to light 15 enemies on fire and to pop the cheevo