How do I defeat the last boss in Lair of the Mad Dragon? I've tried to finished this dungeons with pubs twice during the Dungeon Delves event and both times the group disbanded after a couple tries. The furthest I got was tanking the Dragon on the far left wall and we brought it down to 30%. As the tank, I managed to survive but my group just died and couldn't kill the Eriynes or w/e.

How should this last boss be fought?