Slay Dalgan and Bellandra Felstorm.
The Horde have made our jobs easier for us. Their infighting and segregation have split their camp in two: orcs in the west, and undead in the east.
The orcs obey their commander Dalgan, while the undead follow Bellandra Felstorm, a powerful warlock. Hopefully, their internal squabbles will be enough of a distraction that you'll be able to sneak in there and kill both of them. Sully has already infiltrated the ruins; find him inside when you are done.