Area: Tranquility
Category: Kill drones
Vehicle type: Either
Goal: Destroy 20 drones as fast as you can
Score for Gold: 0:49.80

Strategy #1:

Stand or helicopter hover in place where you start, keep turning and firing.

Strategy #2:

The prison bus is my vehicle/bot of choice for drone missions and melee missions (although I use a flying vehicle for one particular melee mission). The prison bus has high energy levels that don't deplete too quickly in bot form when firing it's primary weapon.

Strategy #3:

As above, use robot mode on a vehicle type with the rapid fire laser. Hold down Y, up, and L. This makes you walk in a tight circle while firing. It doesn't look like it should be as effective as stopping to completely kill the drones in each corner before turning to the next corner, but strangely, it is in fact more effective. You can also wait a second or two when it starts before you press the fire button Y since the drones take that long to get close enough to shoot at.