I found the easiest way to win was to have Lightning on attack Oculi ( hopefully you have this set up from the Nox fight as there is no way to change it until after this fight )
I took out the little ones first then concentrated on the boss, they never got a hit once the small ones were beaten.
I used Finn ( Lightning Bolt ), Aurora ( Slash or Star Light ) and Oengus ( Slash ). All between level 37-40.
Took about 10-15 minutes but once the little ones are gone its a formality. Boss raised another small one but took it out with two hits ( about 850 health )