From Halopedia, the Halo wiki
The Composer vacation is a glitch in the Halo 4 campaign level Composer which allows the player to break out of the playable area and access the battle outside the area where Cortana activates Ivanoff Station's defense system. It does not require any specific skulls, weapons, or difficulty.
- Start the level at Rally Point Bravo
- Play through the level normally until you get to the area where the Covenant is entering the station through the airlocks.
- After you have cleared the enemies from this area, a door will open. Proceed through the hallway until you hit a load zone.
- When you hit the load zone, the door at the end of the hallway will open. Once this occurs, do not enter the room. Rather, turn around and go back to the door you entered the hallway.
- At the door, jump through the door frame on its left side (the side that was on your right when you initially entered the door). From there, jump up on to the top of the door. You can then proceed down the roof of the passageway and out into the open area. There are no kill boundaries, and the ground is solid.
- Attempting to trade weapons with any of the marines will cause both you and the marine to lose your weapons.
- A Cortana model can be found on the ground underneath the room where you activate the defense system.
- Players can jump on top of Phantoms in order to reach and use the Onager. Near its booth, the message "HOLD (X) TO ENTER MAC CANNON" redundantly appears: indeed, the letter "C" already stands for the word "Cannon".