Ok, so I've seen a lot of people on this forum and elsewhere complaining about how to do this, some say they spent over an hour running around and got nowhere near the amount needed. I came up with two methods for this, both worked perfect.
My first effort which took a little more time I will admit, but was 100% successful (came out with more than 90k in damages). What we are looking to do is vehicle damage, this is all that counts, this is where the money is. Now start the case The Drivers Seat, as soon as you leave the police station jump in a car, turn around and drive up the hill, drive down get some speed and hit a car head on, this will cause the car you hit to start flaming, congratulations you did over 1k of damage. How do I know this? because if you go on Rockstars Social Club you can see the cars you unlocked and how much they're worth and if you manage to destroy that car then that counts towards how much damage you just did. If you look through 98% of the cars are worth over 1k. So you destroy their car and your car at the same time, that's 2k. Up and down the hill until you destroy 48 cars to be safe. Too time consuming?
Method 2 -
This kind of follows on from what I was going by before - Vehicle Value. So I was browsing through the car values and looked for the most expensive which from what I have unlocked so far is the American La France Fire Truck $12,459. These can be found at the fire station next to the Hollywood Police Station when you are at the Arson Desk. So, 5 destroyed Fire Trucks Later, it popped (Again! different GT)
Hope this helps, Method 1 takes about 35-40 mins, method 2 around 20 odd mins, those Fire Trucks are tough. Remember, head on and don't count towards your total unless the vehicle is on fire, just to be safe. Enjoy your 30G.