The way I did it was first of all to hide everyone and then ungroup the party...
If you look at the red seen areas, Cazador is completely focussed on the ritual and everyone else is completely focussed on listening to him rant and rave about Astarion, leaving a nice path for you to walk right up to him and take everyone by surprise. Do NOT send Astarion in at this stage, as you will trigger the cutscene and the fight will then take place on Cazador's terms and to your disadvantage. Instead give Lae'zel a potion or two and have her attack him with the deadliest weapon you can get, then action surge and do it all again, then everyone else can hit him with whatever they have - Sunbeam for instance, is a good one. Casting Daylight isn't as good as you might think, as nobody else on his side is actually a vampire. Chatterteeth (a "Skelter" as they used to be called in 1st ed DnD) is another priority to avoid having people Eyebited to sleep. Once Cazador is dead, then you can send in Astarion to sneak attack the remaining werewolves and ghasts. Stand in a group around Shadowheart (or any other cleric) with Spirit Guardians up and watch all the bats fly in and incinerate themselves. Yeeting Cazador over the edge with Force Tunnel doesn't work (he teleports back) as I understand it once did, but you can still do it any other enemies.
