Oh, general purpose damage cuts, element resistance buffs, and reflects stack additively into one term. So if you use a 40% damage cut (like from Joan), then add something like Cupid's +30% fire resist buff when fighting against a fire enemy. That's (40%+30%=70%) reduction of damage. Reiki's summon effect throws another +15% resist against fire on there. Bam, 85% damage reduction from fire.
Eidolon passives like Jack Frost's should stack with resist buffs (but I'm not 100% on this), so double Jack, Cupid's buff, Reiki summon, and Joan's damage cut all together should reduce damage to 0 from fire.