Yeah, I've not got a warlock for the hex in my party. It's a standard party, just a Battle master, thief, cleric and evocation wizard. I can if I focus easily bring her down. Though this gives the AI enough time to group up, and her call for help is her first round go to. So if I don't drop her immediatly she calls for help. I'm doing the path were she tries to heal you, gives you the mark, then takes you into her room. Without the ability to finish her off in the first round, before she acts, it turns into a cluster of epic proportions.
Taking her out with full group isn't that hard, it's the Zhents, and four of her minions that come up spam Aoe's and simply melt my party. You can't heal out of it, because with the action economy, you get 2 heals from your cleric (and SH at the moment isn't very good), then everyone bonus actions for a health potion. Attacks on their turn if their still up. Next turns of enemies are again Aoe's which deal more damage then everything but your Clerics 2nd level cure wounds. Alot of 16 damages popping up. which not even second wind can take multipule shots of that high of damage.
I've not been in that situation myself, I steer clear of marks as a principle. :P
But given the amount of tools you can find opposite the Torture chamber (one million barrels of boom) and the fact that you have an evocation wizard, you should be able to one-shot whatever is thrown at you. I suppose that might be the cheese you want to avoid, if so, I suggest unchaining your party entirely, buffing up Lazael to the point where she should have 20-21 AC is blessed and has a variety of potions (resist this, resist that and haste). Then sneak everyone, except Lazael into the area where the fight is going to happen, far enough away that they wont be part of the actual fight, once you initiate the fight their job will be to take a single action from stealth to nuke down whomever is the highest threat to you while Lazael tank.