Instructions
Utilize the rail gun. Once you have incapacitated Vamp, the second stage of the fight will begin. The screen will split, with you on the left and Raiden on the right. Snake will automatically equip the rail gun to confront the oncoming Gekko, taking advantage of this automatic weapon choice.
Equip the Solid Eye. Using the Solid Eye will aid you in locating the onslaught of Gekko, even through the smoke left from the explosions left behind when they are destroyed.
Charge the rail gun. Holding down the L1 button to aim will begin to fill a meter in the lower right-hand corner of the screen. Once this meter is filled, firing the rail gun will result in one-hit kills. Continue charging the rail gun, even when you don't have a target. Any non-charged shots will merely waste ammo. Extra rail gun ammo can be found underneath Metal Gear REX.
Keep alert, while avoiding distraction from the fight between Raiden and Vamp on the right side of the screen. The majority of the Gekko will invade from the left, but a few will enter from the right so keep sweeping back and forth while aiming to maintain control over the Gekko. Repeat charging the rail gun and shooting Gekko until the fight's conclusion, constantly moving back and forth to prevent yourself from becoming surrounded.