Go into forge mode, place a trip mine, press A and choose Advanced Options.

On min count choose the amount of mines that you have placed on the map. Then go into spartan/elite and keep on throwing them down (don't step on, shoot, or punch the mines).

Keep on throwing them until they blow up on their own.

Warning #1: you will die unless you're invincible!

If done correctly you now exit the map!

Warning #2: if you exit the game, even if you save, when start the game again you will have to redo the cheat.