The boss throws a grenade and a ring appears soon after the ring will fade and it will explode. Kepe in mind the aoe is larger than the circle as the circle is there just to warn you. The circle will appear on a random ally not just anywhere randomly.
Ah see I thought the circle was the radius. Thanks for clearing that up.
To iterate, I have no problems with the boss. Nor am I complaining that its hard, cuz its not (regardless of my lack of knowledge of that aoe). I can see them coming, I just wasn't aware of why moving away still got me hit. Now I know, thanks.