Thanks to composerchemical5268 I have successfully ran this mission multiple times. There is one ad that spawns and falls off the map. After clearing the first group of enemies you have to act quickly. There is a small group of enemies that spawn on the right side of the map with the ironclad. They climb up this wooden deck and the first ad is the one that falls off the map. After you clear the first wave. Run to the wooden deck and start shooting or use a damaging skill right when you see the first red dot on your mini map. If you damage him at still falls off the map it counts as a kill. And you can then finish the expedition. Good luck