Bad targeting issues Zalmarath / Forge Map Boss

There are targeting issues (with certain skills?) with this boss. I'm using explosive concoction and for some reason I have trouble hitting the boss if I'm at a distance. I think it has to do with targeting outside of the walkable area? How about letting abilities default to the edge if our mouse is targeting outside the area?

As of now, it feels awful fighting this boss because instinctively (and logically) I want to put my cursor right over the boss, which is technically outside of the walkable area.

Video:
https://imgur.com/a/vjNJFlx