This trophy is even easier than DirkerLewis describes. You only need to kill all hostiles during the MG mounted truck-segment. Everything that happens before and after this segment doesn't matter. This video from Rooster Teeth really helped me:
Also, very important since this screwed me over for like 20 minutes: shortly after the helicopter spawns (you still have time to destroy it) look immediately to the farthest left. You'll see two 'jumping' motorcycles. The first of them jumps onto the road, you don't have to worry about that one yet. The second one is the annoying one. It will jump and only be visible for about one second. You NEED to kill it at this point! You can still see this motorcycle later (in the last seconds of being on the MG-truck), but it's so far to the right, that your MG doesn't reach him and thus can't kill him. I finally got the trophy after knowing about this.