I did spread my party out. I had my halfling bard, Wyll, Karlach and Shadowheart.
My bard sneaked to Nightsong, freed her and started the fight. Karlach kept Ketehric busy and Wyll nuked everything from a higher position, where I parked him before the fight started. Shadow, who by that time had pretty good armor, just healed and buffed mostly and got rid of the riffraff. My bard used stuff like Vicsious mockery and Tashas Hideous Laughter on Ketheric and bard song on Karlach. The nIghtsong and Karlach did most of the hitting on Ketheric.
I wouldn't say the fight was a breeze, far from it, but everyone survived in the end.
The important thing is: You have to free Nightsong first or Ketheric will be unkillable after a while.
