Here's how I did it.
Step 1: Kill every goblin in that place that you can kill without alerting the Dror Ragzlin room. This is most of them, if you're careful and don't let anyone beat drums.
The drums never did anything in my playthroughs, are they bugged?
Step 2: Go down in the spider pit with Speak with Animals on and make an agreement with the spiders to eat goblins and not eat you.
That's cute. I found out afterwards that you can make a deal with them, and if you open the door they walk up to Ragzlin's door and stay there. Unfortunatly, in my playthrough there was no goblin left for them.
Halsin was bugged for me, too, and only rarely engaged.
Step 4: Put Shadowheart, Gale, and my rogue character up on the rafters above Ragzlin's room, in hiding.
This was actually the tactics for an easy win in each playthrough for me (leaving Halsin aside). Have Gale and the ranged MC (a ranger in my case) pick off the enemies from above, while Lae'zel and Shadowheart fight on the ground, healing via eating their green vegetables as bonus actions. Never needed any other tactics, shenanigans, special effect arrows, barrels, bombs, shovings into spider pits or whatever.
This is kinda true for most of the fights in the EA.