Isn't Exdeath's built-in DC for black magic only? Looks like Cerius and Krile are the only ones that get 'true' dualcast natively (Cerius would be ideal for less squishiness).
/edit: more helpful answer:
Cecil with Focus or Charlotte with Shellga should decrease magic damage a bunch (Cecil can also off-heal and tank the attacks). If you have CoD, she does get a nice resist-all-elements buff but it's not until 6*.
I find physical does more damage than magic, but multiple mages and chaining should do okay. Fire is a safe element to use, although be careful not to use magic right after the "Preparation" phase. If you defend, use auto-attack or abilities it will only do a couple wimpy attacks, but any kind of magic (including "bars" and dispel) results in punishment. Dispel the buff the turn after that.
Is it? I thought it was for all magic... I guess I'm wrong. Either way, having carby on someone for both bar spells in addition to some form of +spr for the party makes the fight a lot easier.