Warhammer 40,000: Rogue Trader

The best way to do this, is do not attack the Chaos Spawn at all until you've killed all other enemies. If you attack the Choas Spawn and do enough damage, it will consume its allies to heal itself. So at the start of the fight position all your ranged people so they have line of sight on as many of the others as possible, preferably behind cover. Use Abelard to tank the Choas Spawn until everything else is down. You will get a little help from the NPCs fight along side you, but don't count on it. Skirt around the Choas spawn with a companion or two if you need to finish off the cultists. Once they are all down, focus everyone on the boss.

And if you feel like having some cheese. Start the fight on easy/custom to kill everything else while taking no damage to your party. Then after you get the Chaos Spawn to under 50% wounds, switch back to Daring and finish it off. However I can't confirm if this works on Xbox, but it does on PS5. I did it the legit way on Xbox, but did the cheese on PS5.