Walkthrough
Simply kill all the Corsairs on the land (the ones on the ships do not count) and then fetch the barrels of oil to light and fire the catapults on both sides. While you are carrying the bundle, click on the catapult firing lever in yellow. Then click it again to launch the oil. Kormir is level 20 and along with the recruits will allow the party to roll over all opposition.

Time is not a factor in this mission, however, if speed is desired, the party can split to clear either side quicker and meet at the barrels.

Notes
If you are a Tyrian or Canthan character, you must speak to Historian Laharo in Churrhir Fields (at entrance to Chahbek Village) to gain access to the city.

If you bring a corpse teleportation skill such as Necrotic Traversal, it is possible to get onto the boat by killing a corsair prior to blowing the ship up.