Just convert immune monsters to absorb

cap enemy resistance at 95%, we have -enemy resistance items.

magic resistant: 10% all elemental absorb.

If it would get the immune tag?

immune: 30% to that elemental absorb.

now sunders are a minor roadblock, we all have sunders again, and actual sunders can cut enemy absorbtion numbers.

fire enchanted magic resistant foe?

takes 5% damage, and only 80% of that.

5k fire damage? x 0.05 x 0.2 = 50 damage.

-10 fire resistance? 150

5k x 0.15 x .2 = 150

just fire absorb sunder? 250

5k x 0.05 x 1 = 250.

-10 enemy resistance + fire absorb sunder?

5k x 0.15 x 1 = 750

fair, annoying, workable.