I imagine that I'm in the minority here, but I've failed thus far in my efforts to eradicate all three of the goblin leaders and consider it impossible.
With a lucky initiative check you can kill Priestess Gut without her taking a single turn. Of course if she does get that first turn and calls for help, things will get more tricky. Minthara is pretty easy, especially if you destroy the war drum in her room. The only problem could be Dror Ragzlin because there are quite a few people in the same room with him, and other goblins will come from adjoining rooms, but some of them have very low HP. A rogue with a little luck on their side can kill three of those in a single turn. The right spell can send a few of them flying into the spider pit. Or you can shoot them from above. There are really quite a few ways to slaughter them.
