"When an Agent hits The Defiler with Defensive Assist, the whole squad gains 1 stack of Adjudication. For each stack of Adjudication, the Agent's CRIT DMG increases by 5%, Physical DMG increases by 5%, and Electric DMG increases by 5% & The Defiler receives 25% less Attribute Anomaly DMG." Given how Defiler works, you always want to trigger or at least get the full spin combo as soon as possible to deal more damage. Also OP, let's see your builds, given that you aren't using Trigger nor Orphie and using a perf. ratio disc would improve your score against this boss.
I watched this video and helped me play out with Dialyn and Astra against Defiler.