Fire deals more damage!

[STM1993]

Positive MP tag doesn't work if the frame isn't activated by button, as YinYin said.
Negative MP tag doesn't work with HP, and it can easily cause bugs like having the burning character suddenly go into standing frame when he runs out of mana.

If you are making a full mod, an easy but very tedious method is to simply make the character have an extra bdy while all fire attacks have an extra itr that can hit it for double damage.

Unfortunately I can't offer any better solution. Trying to spawn opoints with Itr kind 8 cannot be used to directly deal damage, while other Itr kinds could deal the damage alright, but would cause the knockback effect for all fire attacks to be screwed up.