When I finally beat it after 20 tries our group got to the keep and keep 4 people at 1 gate and 4 at the other and 1 of those 4 from each group was on catapult. When the final boss came, we ran up to the King and actually attacked him when he was in range of us, from the kings position, but he was outside the keep. I kept casting spiteful spirit on him and other hexes. After that we killed the last few stragglers and beat the mission.