Choose the Firefight Classic game mode. Go to "Game Options", under "Firefight" settings, and go to the "Custom Skull" settings. Choose either the red, yellow, or blue skull to edit. Go to "Spartan Traits". Under "Shields And Health", choose the "Invulnerable" option. Under "Shield Multiplier", choose "4x Over-shields". Enable "Immune To Head-shots" and "Immune To Assassinations". Go back one screen. Under "Weapons And Damage", set the damage modifier to 300%, melee modifier to 300%, and infinite ammo to bottomless clip. Go back two screens. Under the same skull settings, choose Wave Traits". Set the damage modifier to 0%, damage resistance to 10%, vision to near sighted, hearing to deaf, and luck to unlucky. Return to the "Firefight" settings menu. Under "Wave Properties", make sure the skull you customized (red, blue or yellow) is enabled in each wave and the bonus wave. Save your game type. Change the difficulty to "Legendary". Start the game. You will be practically invincible, and you will blast through the Covenant easily. You should unlock all of the achievements in about three hours, depending on your skill.