Instructions
Receive Metal Gear Rex in the Shadow Moses level of the game. It happens automatically, so you will not have to do anything to obtain it.
Begin the battle with Ray by destroying the Gekkos guarding the tunnel.
Use the missiles in waves by rapidly pressing "Triangle." This will knock Ray down. Dash towards Ray by pressing the "X" button.
Ram Ray when its health lowers. Once you are near it, unleash more missiles. Continue to unleash missile until Ray is destroyed.