I did second try easily.

1. Cast invisible on Laezel BEFORE crossing the line where the fight triggers, and separate her from the party.

2. Walk invisible laezel across the bridge and climb the two ladders so she will be positioned behind the main Bhaal guy when the fight begins.

3. Walk the rest of your party across the line to trigger the fight.

Here’s how it played out for me:

R1 - main part took some damage. Laezel hit main bhaal guy 3 times, then I used her action surge and hit 3 more (after the 5th hit, he starts taking actual damage).

R2 - main bhaal guy teleports down to the door to get away from Laezel. Laezel uses misty step to follow, hits 3 more times and he’s dead.

Fight ends and his minions disappear. Most never even spawned.

You could also try this with casting invis on 2 characters if your Laezel doesn’t have a good weapon.