Maybe shadow heal on top too? And then just tanky enough he always healed back over 50% when hit?
And either Regen or Guardain I am sure.
However, the thing that was throwing me off as far as thinking it was just gear was that the healing was happening when I hit him versus when he took a turn.
Must've been a combination of gear & a relic.
It was a 3v3 fight and the rest of the team was dead, and I was struggling to kill the Pythion before he cycled turns and revived everyone.