While I do have to agree with this, you guys are discounting the light and dark effects of this spell. If a Light magician has cast the spell units get 15+X healing (per second), that is enough to reduce damage from any potential water spell, to almost half. So now we are looking at something like 200X more damage from an ordinary water spell.
Your math doesn't work out. If inner fire heals at a rate about half of the base rate of damage of a water spell, then the net should be 4.2 times normal damage. That's still a large sum.
Moreover, I notice that manypeople still haven't caught on to what I'm saying here: the person with the water spells is the same person casting inner fire. He is purposefully giving the enemy the buff in order to increase the power of his water spells. As such, we can probably presume he'll be using the dark version of the spell so they don't heal, and he'll use it in a situation where the damage buff will not be very useful. I would like to point out that my beef here isn't with specific numbers, but rather principles and applications. I'm pointing out that Inner Fire has a serious potential to be used as a multiplier for water spell damage, rather than its intended effect, if you aren't careful.
I do have one question however: if a player casts BOTH the dark and light version of inner fire on the same units (obviously requires two seperate spellcasters) how does that stack?
Anyways, Reverie will be play-testing and tweaking the numbers, and the end result shouldn't have terrible abuses.