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Building her MAG is easier than her ATK. I think it's best to build her depending on the enemies DEF / SPR. For example, I went to Mad Doll with 800 ATK and 1800 MAG, I could not one shot her with DC Aurora Storm chain. Tried again with 1200 ATK and 1200 MAG, now I could not one shot the small dolls. I went back with 1100 ATK and 1300 MAG, this time it all worked good.
For Fenrir, I had Nyx use Warp Charge and Kingsglaive for break and imperil, Ace for Summon Eidolon, Trance Terras for damage and anyone to summon Phoenix. It has a lot of HP, but any significant fire or holy chains will wreck it in one turn with breaks and imperils.