Originally Posted by
Aidoru
You probably don't need to use D'art at all if you grab Cthulhu. Not that I personally would ever use D'art instead of just using Cassiopea in your scenario. Ryu is 20% a-frame def down, Belphegor is is 12% b-frame and Cthulhu c-frame def down is 20%. That already meets the 50% def down cap, unless that's bugged out too and can go pass -50%.
You might be thinking about atk down being short since Belphegor is only 12% b-frame atk down and Ryu for the a-frame 20% but don't forget water has access to Jormungand as a common support eidolon. It might not be -50% but -42% is still more than enough for most content including Ragnaroks.