*After completing the mission to protect Spargus's gate from the catapult launcher vehicles, I was given this mission. I am not sure if this is different, so I can change it according to any additional information given to me.
Upon completing the above-mentioned trophy, you will be told to make a raid against the stronghold of the marauders. Drive over there using the Hopper, hopping over the gaps in the bridge and you will have to fight sixty of them in their little quarters. After doing so, you will have to chase the stragglers that drove off.
*Hint: If you give them too much of a lead to never catch up in time, just die and you will restart with full boost. The lead between you and them shouldn't be too wide by this point, either.
Drive back over the walkway that has now filled in the gaps and chase after the four vehicles. You will only be able to catch up to one at a time, and they complete what appeared to be a circuit to me. By the time you make it to the last one, the timer will disappear (though if you still let it get too far away from you, it will count as a mission failure).