This is after you have made your way through the Forerunner wall and commandeered the Gauss Warthog. Take the Warthog down the dusty path and complete the following battles with Brute Choppers, Ghosts and Wraiths. As you approach the bottom of the slope, there will be a few Brutes, and depending on what difficulty you are on, will usually have Fuel Rod Guns, Brute Shots and Bubble Shields. As you are fighting them, a few more Brute Choppers will be present behind them, and after a short while the Scarab will make its way down to the battle.