First turn your look sensitivity up to 10.

Activate the music (by holding x on each of the 3 rocks scattered around the map) and when you have found the mystery box then throw a grenade under the box.

Next you will have to use the box, when the lid opens shoot the top of the box 4 times then turn 2 times. With your sensitivity at 10 this should be easy.

Then after 2 spins shoot the same spot 4 times and tea-bag once. This should give you a brilliant gun.