Well, the only other way i can think off is maxing out your dexterity, vitality, strenght and spirit stats with Sources, so that you'll be at least on par with his attack and defensive abilities.
Coupled with Ribbons and Mystiles in all your party elements, elemental-fire to absorb his Ruby Flame, and Elemental-Ultima to neutralize the damage caused by Ultima and Shadow Flare, it can be doable, got to try that setup as well.
*Hits like crazy for 777 damage*!
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
