The chopper assault method is definitely the fastest way - press Play on the "Ride of the Valkyries" cassette tape just before you arrive for a stylish opening. A rapid stealth-based alternative will also suffice, move purposefully to the Walker Gears and planting C4 on each one, neutralizing only the guards that stand directly in you way, before detonating the explosives just before you leave the hot zone.
I got S Rank easily with launcher in this video.
