I was on the boss raid in my clan (Bear Skull), and had my team wiped out after 2nd Odin attack, except for a Sentinel Banshee (lvl 93!) with a newly installed Cosmic T1 talisman. The Banshee proceeded to take Odin VII down to around 25%, then her hits started giving him health, all the way back up to 100%. After half an hour running on auto, I finally quit so I would stop healing Odin and reversing any damage the rest of the clan was putting on him. Banshee put nearly 30 Million damage on Odin by herself, but he was not feeling like dying today.
EDIT: Looks like the bug is the Banshee & Cosmic as I get the same results in arena.