Granted, this boss has the MOST MINOR CHEAT EVER! A Minor Blessing heal interrupt. However, I believe that I have identified the counter to his cheat. If anyone else has already posted this, I didn't find it when I searched.
Kallah Silverback in the Drum Jungle says, "Your unaltered spells won't even scratch my fur." and "Your school magic barely tickles me." when attacked. Casting a prism before attacking causes him to say, "Oof. I wasn't expecting that."
To avoid his Minor Blessing interrupt, just cast a prism before your single attack.