Go to the bridge that takes you to the second island and has train tracks under it. Walk along the train tracks towards the second island before you are allowed to cross the bridge. When you are about half way, you should have a six start wanted level. Then, just stand on the train tracks for five minutes to get the achievement. You do not have to worry about the police because they will be on the bridge above you so they cannot shoot, and the helicopters are above you so you have cover from them.
Go to the hospital at the north-west corner of Algonquin. You can also get there by dying in that general area. Get a motorbike. Set a Waypoint at the hospital or just remember the way you came. Drive to the nearby bridge that takes you to the third island. You must not have access to the third island for this trick to work. Slowly reverse backwards over the barricade on the bridge until you get a six start wanted level. Then, drive straight back to the hospital. To the right of the hospital as you are coming back is a little red fence that you can bust through and get onto the elevated train tracks. Wait there for approximately five minutes to get the achievement. No cars can get to you on the train tracks and the helicopters cannot shoot accurately enough to hit you at motorcycle speed.
Attempt to enter one of the locked boroughs early in the game to get an immediate six star wanted level by taking a car on the subway (or elevated train) tracks. This can be done easily near the corner of Grand Boulevard and Leavenworth Ave. in North Bohan near the police station. From Leavenworth, take a left onto Grand, and go up the little hill. When you reach the crest of the hill, look to your left to find a very short wall above a subway tunnel. Drive a car over that short wall, then drive underground toward North Algonquin. When you have a six star wanted level, stop your car and remain there for five minutes to get the achievement.
After the middle island is unlocked, steal a helicopter from the helitour site in the southeast corner. Fly over any other area you have not yet unlocked to automatically get a six star wanted level. Fly off the map in any direction, and stay in the air. The police helicopters will only chase you so far. After five minutes, you will get the achievement.