Dev, check out GuildWiki for the Thunderhead Keep Mission. I like this strategy - Lighting Torches for bonus.. I was outside with Digger and saw that location.. Sounds like it might be easier to try. I could lay traps before they spawn. It would be easier then getting surrounded on the stairs. Might be worthwhile to try. hhhhhmmmmmm
Strategy C
A new technique that is becoming more frequent begins similarly to strategy B. The party initiates the bonus by lighting the torches, then defends near the king's throne. However, after the initial several waves of attacks, there is a break in the battle. At this point, the entire party heads outside of the gates and proceeds north, ensuring that no creatures were already enroute to the keep from either the east or the west (note: you can safely ignore the Murssat and Jade that are in the Dwarven villages, they will not attack unless you approach them).
All enemies that attack from this point onward spawn in an area to the north of the keep, near the portal that later transports everyone to Ember Light Camp (Note: the portal only works in the cut-scene, walking through it prior to completing the Thunderhead Keep mission will have no effect). To reach the keep from the spawn point, all enemies must pass a narrow choke point. The party can station themselves at the choke point, and eliminate all attackers without risking the King engaging any of them. Dorian's party also spawns at this location, and can be eliminated in the same way.
Note that just below the choke point are two parties of Jade. These can optionally be ignored, or destroy them during breaks in the spawns so that risk of accidentally aggroing them is eliminated.