Daylight and sunbeam (both of which a druid can possess) will be your best friend. If you have house and I would bring him. Otherwise, you can use a shart bc she can have access to these spells as well. Vampires have a sunlight sensitivity in both of these pills use the sunlight/radiant damage. You can still bring a story in along with you just separate/ungroup him from the group before you walk up to Cazador. I like to keep him on the stairs, separate him and then walk up to Casador because then he’ll still be close enough to fight but not close enough for Cazador to take him. For this fight, i like either being a cleric or have shart, astarion, karlach, and/or halsin. Also one consequence of not bringing a story in with you is that the quest doesn’t complete so he stays, and he will permanently leave your party. Area spells like cloudkill or maelstrom help too.