It's hard to describe but if you go up to the top left front of the goblin camp, there are some drunk sleeping guys - just past them is a wall you can break. That lets you into the upper rafters of the goblin base. You usually have to kill the drunk guys but using a silence spell whilst you smash the wall would probably also work. Once in, you can work you way across various rafters all the way to Droz Raglin or whatever he's called, who is giving a speech. I think proceeded to throw barrels of smokepowder, spears, and other heavy objects down on him and his followers, whilst keeping a Cloud of Daggers on the most obvious ladder up to the rafters. It went pretty well! Lot of explosions, lot of knocking/throwing goblins off the rafters as they came up. Unfortunately I did have to reload my post-battle save, because I forgot one of the barrels of smokepowder I threw didn't go off (no fire) and had the party walk over it, including Gale, with a naked flame (a torch). Ooof. There are probably other ways up into the rafters and whilst they expose you to some fire (but the shooters are penalized due to high ground), they give you a huge advantage in throwing things (you get the bonus "object fell from a height" damage which is kind of huge on something as heavy as, say, a spear) and constrict enemy paths very well.