Recommend you do this in 'Big Team Battle' as there are plenty of vehicles around and the Skewer spawns in one of the weapon bays.
I got it shooting down a Wasp, but
Toddillini mentions it works on the stationary AA guns and will be the easiest way to get this.
Warthog or Razorback will take 2 hits, while Mongoose, Wasp, and Banshee will take one.