Sometimes he goes unattackable/untargettable for a few seconds. One time he walked all the way back to his spawn point before I could re-engage.
He used a rogue-like escape, disengaged and walked back to his original spot. Frequently. He has a nasty knockback. We were trying to pull him into a nearby room, but he would be 5% healed or better by the time we got him back. Very annoying fight. It worked better to keep our backs to the wall and kill him where he stood. Needed some crowd control to mezz the roamer. Dots didn't seem to clear when he escaped and did some of the best damage overall.