Quote Originally Posted by lolix View Post
How exactly does resistance work ? Like a % damage reduction from a certain element ? And does it stacks with other damage redictons (like jeane's ) -- And if that's the case , aren't resistance up eidolons pretty damn good (even if specialized) ?
Yes, yes and yes.

Using double Jack Frost gives you -20% damage taken from Yatagarasu. Adding Joan on top of that and you'll have -60%. At least, as far as I know. So yeah, resistance up Eidolons are unmatched as long as you know what element you'll be going against.
Quote Originally Posted by lolix View Post
or other defense increase buffs ?
... kinda, but unlike damage cuts, they're not added. If you get 100% Def up, you'll take half damage. So if you're using double Jack Frost and using Joan's damage cut on that turn, then 100% Def up lowers your damage taken from 40% to 20%.

But you'll never reach 100% Def up, so in general, it's a crappy stat. Better than nothing, I guess.