For a bit of clarification, the achievement is asking you to do a vehicle takedown while flying a Buzzer. The trick to this is that you must hover the Buzzer next to the driver or passenger door on the vehicle you're highjacking and push in the Right Stick. Easier said than done, but not impossible.
If you do not have the Hurk's Redemption pre-order missions, you'll have to try for this without a mission to give you the set-up you'll have to wait for a certain mission in Act III before this will be set up for you (not giving details to avoid spoilers). If you'd rather get it before Act III, then follow the solution.
Obviously, you will need a Buzzer. One can always be found at Khilana Bazaar, the very first story-related Outpost you took over. It's where you first make the choice between helping Amita or Sabal and where you encounter Hurk for the first time. There is also one available for purchase as an upgrade to the Ghale Homestead, if you've made it that far into the campaign.
At any rate, once you've got your Buzzer, now you play the waiting game for awhile. Fly around the map, especially in places that are fully or partially in enemy control. You are looking for a Royal Convoy truck to hijack (not the Pagan's Wrath version as they will shoot you down). When one does finally spawn on the map, it's got a nice big icon of a white, onward-facing truck with a light blue hexagonal frame. It'll even float at the edge of your mini-map to show you where to go. Follow the truck as best you can, as it'll likely be faster than your Buzzer. The driver will spot you, but more importantly he will not fire at you, so the Royal Convoy are ideal targets.
In general, these trucks will drive in a circuit so you can keep following, cutting corners to catch up. Occasionally, the truck should slow down for a short time for a difficult turn or whatever, and may possibly even stop, during its route and this is what you're looking for. Stay with the truck until it slows and then fly up level to the driver's window (apparently the passenger side works as well) and push in the Right Stick for a vehicle takedown.
You will not be prompted when to do this, so don't wait for a button to appear on screen. As soon as you get the Buzzer anywhere close to the window of the truck, start punching the R-Stick repeatedly and eventually it should catch and go into the takedown animation while simultaneously unlocking the achievement.