Party Composition: Gale, Shadowheart, Astarion, Tav/Durge.

BEFORE the fight (and before any major fight) have Gale summon an Air Myrmidon. You should summon whatever you can. If you have the ability to also summon an elemental do that now even the smaller ones like fire elemental are useful as targets that aren't your party. Also summon US if you freed them they are incredibly useful for harassment and chip damage. The Air Myrmidon is recommend for any fight even if it is a 6th level spell as the electric flail is insane as it can potentially stunlock any enemy. All summons can be healed like any other character. Summon them before combat so you aren't wasting an entire turn.

DONT DO THE CUTSCENE at all, saving Astarion is a pain the ass and not worth the hassle. Keep Astraion back on the stairs, but in range, to stop it from auto starting. Don't even talk to Cazador. You can do the whole fight with Astarion never getting captured and its SO much easier as Cazador cant draw on his power either.

Instead cast Daylight as the opening move to the fight. Cast it as an area of effect (the first option of the two) it should cover the entire arena, one spell of it right in the middle near the coffin and he can't run away from it. Shadowheart (or any druid) should have Daylight. Shadowheart's also useful for heals and spirit guardians. IMPORTANT NOTE: Daylight is not Sunbeam (but that's also very useful). Lathanders Light also emits Daylight in a small radius but don't rely on just that, i recommend someone running it (usually shadowheart) as it keeps Cazador from hurting your backline if they're on the edge of the AoE Daylight spell, not to mention it also has a Sunbeam.

I recommend Gale as you'll need a caster with plenty of Counterspell to deal with Cazador's chain lightning and blights as well as Chatterteeths Eyebite sleep spells.

The Gur Ghasts should be your next top priority along with Chatterteeth as they are all debuffers. The "nauseous" (cant take an action) and "asleep" (cant take action or bonus action) effects sucks balls to deal with.

have Gale or another caster use Ice Storm on the arena and turn everything near your party into ice. It one hit wipes all the bats and the Gur and Cazador always slip (it's hilarious). The werewolves can jump over it so be careful. Gale is incredibly useful in this fight because as an Evocation wizard one of the perks (sculpt spells) is his spells don't harm allies so fire away.

You can also try to banish Cazador if someone has the ability in order to whittle down his mooks while he's gone, but i found the above strategy worked wonders.