Hmm, weird, i never use mage more than once in CW though, and i barely recall how good my black hole back then, but your int seems higher than me.
Anyway, i guess the weird thing is a good weird thing right?
I mean doing more than the listed dmg.
Just a theory, maybe the listed max dmg in the description can't go over 19k, but your spell can exceed that number. Or maybe boss battle has its bug, if i remembered right boss is the most buggy elements of CW