This is the messenger you have to steal notes from in Act 3.
After a few attacks, I figured out the idea behind it all. The messenger summons meteorites and teleports away if you get too close. So I stand in the spot he's going to next appear, wait for a red circle to appear and then rush towards him. The idea being that the meteor will hit him.
However, the messenger has no health bar. Also, he doesn't do anything to indicate a hit. I can't see if it's having any effect. How do I know for sure it had effect? Does it really need to be spot on?
EDIT: The solution appeared to be using Yrden to trap him in place, they slay him. Here I was thinking the developers had come up with a nice trick of using an enemy's weapon against him.