Need help killing Balthazar in the Nightsong's prison

So I'm on my first playthrough, recently got to the Gauntlet of Shar and started doing the various quests as you'd expect. I wanted to kill Balthazar, but I remembered Z'rell saying something about "death wouldn't silence him for long", so I decided not to.

Problem is, now I have to. I took all the orb-things (including the one Yurgir guarded, using stealth instead of combat) and completed the library. Balthazar entered the Shadowfell with me and of course he wants to take the Nightsong, as y'all surely know already.

Now here's the problem: I put roleplay over making my life easier. So I got to this point in the story skipping some quests and I ended up vastly underleveled - I'm at level 5 - when I got to the Shadowfell. By some miracle, I managed to get this far into the game, but this fight simply seems impossible. Even if I put the game on Explorer mode I still end up dying.

Now, I am a D&D player, so I know the rules well enough to understand this game pretty well, and I'm willing to do a bit of savescumming, but I don't fancy loading a save from hours ago and changing my choices.

I've seen people saying to push people off with weapons, the shoving action and spellcasting, but I don't know how viable that would be as a tactic if at least one of my characters get annihilated by Balthazar in the first round.

Help?

TLDR: Need help killing Balthazar and his minions in the Nightsong's prison with a level 5 party.

Side note: I still wanted to loot a couple of objects in the area around the mausoleum, will it be possible after this fight?