Frost Gate bonus?

Where do you get the bonus in Frost Gate co-op mission and how do you complete it?

When you enter the Dryder caves, go left (East) instead of right and talk to Rornak Stonesledge all the way at the dead end. He gives you the bonus mission.

When you exit the cave...keep RIGHT and you'll see a path open up down along the cliffside that takes you to an empty ballista. That's the one Rornak speaks of. Firing it blows open a door that you can now go through.

If the dwarf dies however you can't complete the bonus, so the best idea is to clear the path to the ballista before you talk to him (can be a pretty tough fight).