Use the following trick to get an unlimited Spartan laser on last level "Halo". 
Play through the level until you meet up with Sergeant Johnson. Shoot him until 
he falls down and drops the Spartan laser. Quickly pick it up and shoot it at him.
Keep shooting him until he kills you. When you respawn, knock him down again and 
pick up his laser. He should get back up with another laser on his back. When he 
does, shoot him down again and pick up the new gun he drops. This laser should 
have unlimited Spartan ammunition. This makes it easier to destroy the Guilty 
Spark 343.