Shoot yourself into the middle of the city or through the map
Go into Keys Of The City by yourself or in Co-op mode on Xbox Live. Put four stars on all your skills. Go on the highway. Have a friend drive the Agency Supercar or do so yourself. While you are in the car use the cheat window to spawn a ramp truck. When you are at full speed hit the spawn ramp truck button about three times. You will fly or go through the map. If you spawn a ramp truck on Wang Tower on the cement wall sides then jump off it with an Agency Supercar, you can go through the Wang building as well. If you get stuck you may have to extract.