Just had to fight a legendary tracker orc, who was impenetrable to arrows, had vault breaker, and had a pack of the spear throwing orcs with them. And he kept putting little traps on the floor. Man that was frustrating.
Sometimes the game can be a bit too easy, sometimes it throws the most over powered captains in there to really test you. Didn't help that i hadn't killed his bodyguard so had to deal with a poisoning berserker guy at the same time though. Ended up winning through a combination of climbing buildings and beating the crap out of them bit by bit over a very drawn out battle. Even managed to steal his bodyguard as a captain for my army