I can chime in for this now! I had a silly strategy, pretty much the same for the Elder Bunyip.
Party was Nolan, Gregory, Isaiah, and Bridget. Isaiah was the lowest level (79) and the others were around 85. Isaiah had the protection build, which was his main purpose in this fight. Have Isaiah defend, tank all the hits for little damage, have Nolan cast Deterioration for continuous damage on the DQ then Chainsaw her every turn, Gregory cast Ebonflame on DQ, and Bridget buff party magic defense, remove poison on Isaiah, and do continuous healing as needed. Between Nolan and Gregory, I'd hit DQ for about 20% every turn, so she wouldn't cast a spell or attack and it would just trigger her Phalanx Fighting buffs every turn instead, so she never had a chance to seriously endanger the party.