Personally, I would go for a more story based approach, allowing
clever Helerings (who I would classify as using mostly Water magic)
to somehow claim that their rain helps overcome fire as if it were
storm because ....then some hand waving about how Heler joined the
Storm Tribe, how he learned the ways of Thunder etc. I might even
require a roll to justify said handwaving. But, to me, Rain=Water,
Lightning=Fire, Thunder and Wind=Storm, Gravity and Soil=Earth,
Shadows, Cold= Darkness.
Keith