I decided to post this separately because this is about mystics in general. If DEF stat offers magic resistance, I think it is fair to make the same case for Mag stat. Mag stat offering physical resistance would offer mystics a great buff that doesn't directly nerf ranger squads and any other trigger squad that plague the higher tiers but also would not change how a mystic is typically played. Maybe making 33-50% of the MAG stat be added to the DEF stat when calculating for physical damage is a good number.
Edit: I realized 33-50% would make mystics highly unkillable esp. RM Kiera, Ronan and ele Zoey. 25% is actually fine. It makes crits from rangers still hurt but I think would now require rangers to proc 5-6 strikes to kill a bulky mage.
Re: make kiera great again?
My Kiera can have up to 26k MAG due to buffs. My only lament would be the fact that she can not one-shot all the Guardians all the time. Most of the time she'd need the burn to proc once/twice before the guardians are wiped out. This is not an equal exchange for the effort and time needed to keep her alive till she can cast her Sacred Blaze. There are even Heroes like Leah Warlord who can wipe out all my Healer/Mystics with one-shot even tho they're units pumped with MAG. Kiera is a Valiant, not some side kick of a hero...
