Against the hobgoblin boss, I killed after clearing minthara. You'll see there's a Battle drum near them, make sure to have someone to break it so they don't summon the whole camp. I set characters that could long range in the rafters at the very top. They were still passive toward me so I was able to move around pretty much anywhere.
Me and my friend have a party of 2 warlock 1 cleric and 1 fighter. I put my fighter at the drum and smashed it down. I started the fight and my friend used his warlocks to cast witch bolt at the hobgoblin. We kept our casters above the boss so he wasnt in any danger. They would sometime run toward the ladder so we had a grease bottle spread at the top of it. When there was enough of them going for the ladder we set it up on fire. Having a gunpowder barrel or a oil barrel can help a lot too because it'll explode when it hits the burning area and cause a lot of damage. I know you are supposed to be able to knock the braziers by knocking down the wooden thingnies holding them up so you can also spread a Lot of fire field that way.
Hope it helps!
