1) Load up the mission "Little Resistance".
2) Play through the mission until you see a friendly tank moving towards the beach.
3) Get behind it and follow it onto the beach.
4) When you are on the beach will find a small puddle at the right side of the beach.
5) Jump in it and stand in it for a couple of seconds.
6) Jump out and run to your left till you see another puddle.
7) Jump in it and stand in it for a couple of seconds.
8) Go to the far left of the beach and jump in the puddle with the barbed wire.
9) After several seconds, the screen will shake, and you will hear strange voices.
10) Four lions statures will slowly arise from the ground.
11) Go up to one of the lions and you will notice a Ray Gun in its mouth alike the other three.
12) Hold "X" to take the Ray Gun.

Here is a video tutorial: