Instructions
Go to the top of the tower in Halo, where Sergeant Johnson is firing on the Flood from a cliff. Do not kill the Flood yet.
Knock Sergeant Johnson off the cliff with a rocket launcher or grenade; he will drop his Spartan laser as he falls.
Pick up the Spartan laser Johnson dropped. This is not the infinite-ammo version, but you must pick this version up to get the infinite-ammo version to spawn.
Kill Sergeant Johnson, who will not stay dead. When he comes back to life, he will possess the infinite-ammo Spartan laser.
Kill Sergeant Johnson again; he will drop his new laser.
Pick up the dropped Spartan laser. Although it displays a charge of 100, the charge does not deplete when the weapon is fired, allowing the laser to be used indefinitely.