The comeback kid + leech combo should be nerfed.
Anyway, I can heal my whole party almost instantly by casting fireball on them, because they have very high elemental resistance to fire. Meteor shower is the best though, I can watch the enemy burn to ashes while my party heals while basking in the extreme heat. My entire party actually has 100+ percent resistance to every element, so even breathing in poison heals them too.

...That is much worse than leech, but nobody seems to care. Strange how that works.
Because it requires a significant investment both financially and in points spent.
It is extremely easy to get 100+ elemental resistance to every element about mid game.
So you're saying that leech is a problem but getting healed by every elemental attack isn't? I'd say that is a much larger problem than leech, because it makes the game way too easy. If anything needs to be nerfed, then this would be it.
Leech isn't very powerful anyway, it only heals like 15 to 30 hp and is worthless against crushing attacks.
It is fine. It is again an NPC problem and not an mechanical. The AI will not remember that targets are immune to their elemental damage, so they will not try other damage. Furthermore they will not just rotate stuff like nullify resistance.
We have no PVP in this game, so the game does not need to be fair to NPCs, but smarter NPCs would make more interesting gameplay. Right now the AI is very bad at later stages of the game. TO much options, not enough brain for them to deal with it. A leech character is in some encounters invincible, not because of leech, but because the AI will insist to heal them up constantly.
Same applies to elemental immune characters in other battles.