This mission was a pain but thankfully I past it. Here was my strategy and I will try to go into as much detail as possible. Go for the first 3 courier on the left side up ahead of you. Come to a complete stop about 2 car length away behind the closes courier which is a van and start firing away. Turn quickly around and make a right, the next courier (a dirt bike) should be up ahead. Use the same strategy as the first one because the first 2 couriers will get off/out their vehicle and start shooting at you. Kill him and quickly fly on to the next courier (a van) a little north from there. Get in front of him and start shooting as soon as he gets within shooting range. The reason why I get in front instead of behind is that the last 3 couriers do not get out/off thier vehicles to shoot you. Instead they continue to drive faster in a forward direction (especially the 2nd dirt bike) while shooting at you if you were behind them, which will be harder if you don't blow them up quickly because you'll have to catch them. The next one for me was the dirt bike. I used the landing in front strategy for this one too. The last one, you can either land in front or in back because it doesn't move as fast. To easily past this mission, the 2 last courier has to be somewhat close to each other and that will vary each time you retry the mission. It worked for me, but it may or may not work for you.
