Area: Tranquility
Category: Break stuff
Vehicle type: Either
Goal: Get 50,000 destruction points as quick as you can
Score for Gold: 0:49.19

Strategy #1:

Very straightforward. Stay in one place, move around, doesn't matter. Just shoot at and crash into everything in sight. If you want to be particular, vehicles that shoot at you are worth more than civilian vehicles are worth more than objects.

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. Stand where it starts you, hold down the fire button, and use L and R to mow down the vehicles that approach you.