Prototype

This is somewhat easy. You can steal a tank and then drive around the city looking for helicopters flying in the sky, or just cause an alert while in a tank and let the helicopters (and fresh tanks) come to you. Either way, lock onto the helicopters with L2 and fire when you have a clear shot. It shouldn't take much to take them down. You should be able to take out a large chuck of this trophy, if not complete it altogether, while working towards the Threat Elevated trophy.

You can track your progress under the Statistics -> Vehicles page on the pause menu.