First off you have to start from the beginning of the level Halo.

Go all the way through until you have to go through the final door in the building to reach Spark.

But, don't go through the door right away. First, kill Johnson.

Then, before he stands back up, run to the door. Kill Spark, and if Johnson didn't stand up before you opened the door, than he will still be alive.

Kill Johnson again before the flood can get to him. He will drop his spartan laser.
Simply pick up the weapon, and viola you have a spartan laser with unlimited ammo.

Note: you can kill Johnson repeatedly, and he will always have another spartan laser unless he is infected.