I agree with Gogeta there. Also be sure to have lots of SMG ammo to issue drive-by justice on the way there, particularly to any Stingers that may try to catch up. The route is -
Exiting Cochrane Dam, first left.
Follow that road as it goes all the way down, straight past the airport and up to the bridge, then back down to the Staunton Island West drag.
Finally, turn left toward the Callahan Bridge and City Hall should be on the way.