Find the bridge that takes you to the second island and has train tracks underneath. Walk along the tracks toward the second island, before you are allowed to cross the bridge legally. When you are about half way, you will gain a six star wanted level. Stand on the train tracks for five minutes to unlock the trophy. The police aren't a problem because they will be on the bridge above you and cannot shoot. Their helicopters are above you so you have the bridge as cover from them.
Go to the hospital at the northwest corner of Algonquin and get a motorcycle. Set a waypoint at the hospital and drive to the nearby bridge that takes you to the third island. For this trick to work you must not have access to the third island. Slowly reverse backwards over the barricade on the bridge until you get a six star wanted level, then drive directly back to the hospital. As you are returning, break through the small red fence to the right of the hospital and get on the elevated train tracks. Wait there for five minutes and the trophy is yours. No cars can reach you on the train tracks and the helicopters can't shoot accurately enough to hit you when driving the motorcycle at a moderately fast speed.
Try entering one of the locked boroughs early to get an immediate six star wanted level by taking a car on the subway (or elevated train) tracks. This is easy to do 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.
After the middle island is unlocked, steal a helicopter from the helitour site in the southeast corner and fly over any other area you have yet to unlock to instantly get a six star rating. Fly off the map in any direction and stay in the air. The police helicopters will only chase you so far. You will get the trophy after five minutes.
Unlock the "Add one star to Niko's wanted level" code in your cellphone, then go to any one of your safehouses and activate that code six times. Wait five minutes, then enable the "Remove Niko's wanted level" code to unlock the "One Man Army" trophy. Note: Doing this will block the "One Man Army" and "Walk Free" trophies.