"Firestarter", "Blaze Of Glory", "Heat In The pipe", "Game, Set, Match", and "Crowd Control" achievements hint
-On Firefight Classic, go to "Game Options" and under "Firefight" settings, go to the "Custom Skull" settings. Select either the red, yellow, or blue skull to edit, then go to "Spartan Traits". Under "Shields And Health", select 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 and "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 and change the difficulty to "Legendary". Start the game. You will be pretty much invincible, and you will blast through the Covenant easily. You should unlock all of the achievements in about three hours, depending on your skill.
-Note: To do this much easier, follow everything in the above hint. However, in "Wave Properties" in the "Skulls" menu, enable every skull to every single wave--including the bonus wave. Then, choose "Bonus Wave" and set all the enemies to "Hunters". Set the timer to 2 minutes (because Hunters give the most score for taking them out). On the first page, set the "Wave Limit" and "Time Limit" both to "Unlimited". Set the "Game Difficulty" to "Legendary" and start the game. Once the game starts, take out enemies until you obtain an explosive weapon (concussion rifle recommended). Then, start using that weapon and fight off all waves. Once the bonus round starts, there will be three Hunters spawning each second non-stop until the map cannot spawn any more of them. If one of them gets killed, it will instantly get replaced by another one. Keep rapidly pressing RT to quickly take them out so they can be replaced fast. You can quickly kill more of them and will be rewarded with high points. Repeat the process and you will get about 1 million points in just two sets, which requires about 40 minutes.