Divided by 3 would make it a more expensive clone of Fire Bolt. Making it half would just make it very similar. You should make something completely different. Example:
Spontaneous CombustionCosts 9
Deal 10 damage to the target. The target is dealt 1 burn damage with every attack. Stacks.
So, using it on a player with 100 health brings him down to 90. Let's say you have 3 Ash Eaters on the field. Normally, they would do 2+2+2 damage for 6, bringing him down to 84, but now, they deal (2+1)+(2+1)+(2+1) damage (the 1 being additional burn damage), or 9, dropping him down to 81.