Cap'n Kobold
Hero
I actually think that there is more character development potential in her learning that fire just plain doesn't work against some things, and her having to come up with alternative solutions.Elemental Master is intended to be used by my completely ridiculous PC, Flaming Helen, who tries to solve every problem by burning it with fire.
The character was a 3e warmage with the Fiery Burst reserve feat from Complete Mage and would be a draconic bloodline sorcerer in 5e.
Failing that, I think treating immunity as Resistance would be better. Or converting some/all of the damage into a different form. A "Holy Fire" that converts half into radiant, or "Overpressure" that converts half into bludgeoning.