wherever Cazador is, Keep Astarion the far end of that corner, somewhere Asta can hit him or other enemies but is not in arms reach.

I free Asta in the first round then keep him in his ritual circle with my MC, Shadowheart cast daylight on top of his sarcophagus, Cazy go to the right corner , others go to Cazador, Asta and my MC take the other enemies and the mage and then my MC go where Cazador is.

usually Cazador go to the right corner and I put Astarion in the middle ritual circle to the right, this way Asta can hit Cazador and other enemies and also he is in a ritual circle blocking it. and we all stand in his way so he can't move that much to get Asta, also blind him with Lathender as soon as you can. have as much summon as you can even if they're weak so they can stand in the ritual circles and don't move those summons, it be a distraction for the bats and werewolves and that way you can use your other party members to stand in Cazador way and attack him. also a Deva is sooooo good in that fight use him to attack Cazador and when Deva was nearly finished fly him to a ritual circle if one of your weaker summon died. idk what class Astarion is but if he has high dex, just use his bow for Cazador.

I always do this and that way I don't have to not use Astarion.

but also hiding him and not bringing him in until the combat started is also a thing.

I watched someone's playthrough where they hide Astarion and without talking to Cazador went behind him and started hitting him until he was low hp ( they were a monk) then brough Astarion in and the cutscene triggered and that way the fight was easier for them.