The lights are on
The process is very simple.
The first thing you need to do is just allow the enemies to pile.
Don't switch out any of your basic already carried on weapons. Leave them.
Wait for piles and piles of enemies to come after you. Or you and your partner if your playing Co - Op campaign.
If you run around the base of the area which you are at. A Spartan Laser will pop up, DONT TOUCH IT !
Go behind it and then you'll see your Rocket Launcher.
( This works on all sections of Halo 4. )
It may or may not work on Multiplayer because I haven't tried it yet.
Thanks for reading !!