Are you sure? The fireball numbers seem to add up, and both skills display the enemy resistence as -100%, and a damage number that is twice the one shown in the skill tree, so I would assume it is already being accounted for.
Besides, if 56 k is the base damage (before resistances), then it should be 56 k * 2 * 0.71 =~80 k


Fire Ball – Level 37
Damage: 27,384–29,806 (showing 13.6k - 14.k in skill tree, stating -100% res in calculation. seems ok)
Cast Rate: 8 frames (3.13 casts per second)
Damage per Second: 89,360 (which is 3.13 times the damage, also seems ok)

I also don't see how the hydras summing up to 500k dps. They would have to be attacking almost 10 times per second, each

Hydra won't benefit from all the reduced fire resistance gear except infinity

Good point! So that number is definitely broken

Still looks like it's going to be a solid endgame build though. I'm hyped for ladder reset

The damage you take from fire, especially when using v magi and it’s MDR is going to be negligible given most fire damage in the game is dealt in low damage, a lot of times, per frame.

With anya, 20 torch , 20 anni, 35 v magi, you are already at +5 fire resist. Not counting the resist on amulet or small charms. That’s not even counting using aldurs or tri res boots.