Disc Priest Help

Disc Priest heals by keeping up Atonement and doing dmg (or directly healing). While questing in Stormsong Valley, Kul’Tiras (BFA), I came across a miniboss (don’t remem name) around Mariner’s Strand… he transforms into a faceless and uses shadow/void powers to healbk in a similar manner to Disc priest. I ended up in an infinite healback loop where he could not kill me but I was not defeating him either. I wasn’t sure how to outrotate his abilities. What am I missing?