I did this cheat with a helicopter from the helitour in Algonquin and Alderney not unlocked, but if you have a helicopter cheat and an island or two locked, this works better.

Hijack a helicopter and go to a locked island in it. Go over it to get the wanted level.

From there on out, keep moving and alternating heights to get the achievement.

The reason why you must move and alternate heights is to avoid being shot down.

I suggest moving far away from that island so you have a better chance of surviving and not having the police stay in one spot of the city trying to shoot you down.

So alternate heights greatly (but stay relatively high to avoid breaking the helicopter) and move all around city.