[IronDragon]

Do NOT use cloudkill. It will take out that prisoner and do horrible things to your reputation. This individual plays a roll in the thief stronghold so it behooves even evil parties to keep him alive.
A fully charges wand of monster summoning is the best weapon here. Stay at the stairs and start casting. Five Gnoles will clog up the doorway and the bad guys will have to go through tem. Keep the door way clogged and only one or at worst two will get through for you to fight personally.
Use any and all spell protections to keep yourself alive. Yes the heavy duty arrows and bolts to hammer away at the bad guys trying to get through your wall of summoned monsters.
If your mage can lower resistance or cast greater malison use those and a wand of fear to even out the odds a bit. Take out his familiar imp ASAP to knock a few HPs off Mae'var. Since you have a swashie and a bounty hunter use their tactics against them and do some backstabbing of your own.
Just keep the potions of healing handy and don’t stop hitting him.