Getting past cazador is a lot easier with alert because it basically blocks his surprise moves and gives you first dibs on attacking. The way initiative works here with the d4 roll means that +5 bonus is actually huge, and you'll pretty much always go first which is exactly what you want against this guy.

The really nice thing is you won't get caught off guard during his ritual stuff, and if you've got astarion with you that's super important since cazador likes to try and trap him. Once combat starts you can jump right in and start messing up his plans especially if you brought some good daylight effects or radiant damage spells with you.

Tip for you though - alert only works on whoever has it so your other party members can still get caught with their pants down. Might be worth considering throwing some moonlanterns around, and definitely pack stuff like sunbeam if you've got it since those really mess with his whole undead thing. The key is acting fast and controlling the fight before he gets a chance to do his thing, and alert pretty much guarantees you'll get that chance.