This is based on my experience, so I apologize for any confusion this may create. After completing the objective "Race for more artifacts", in which you would receive the following trophy, you will be given an option to hop into one of the vehicles near the front of Spargus to protect it from marauders.
This simple mission will require you to destroy the catapult launchers that are coming in (about 13, if I remember correctly). You can focus on taking these out exclusively, as only one or two will usually be around at one time (mostly one, though you might find two later near the end). Regular vehicles and even flaming, kamikaze carts will be running around as well, but as long as you focus on the catapults, you will be able to do this with little difficulty.
*Immediately following this mission was the mission for: