Huh, surprised that ~31k with 60% assault with a team of Cassiopeia (Sniper Shot), Gabriel, Triton, Nike, Rusalka, Dazbog (was leveling her, forgot to swap her out), and Undine (without 2nd skill unlocked...) got Ragnarok down to about 10%.
Swapping out Dazbog for Kushinada, picking up Undine's 2nd skill, and refining strategy should close the gap. And maybe some more time/gems thrown on raising that assault more.
The healing block/burn is pretty vicious though.