A Monument To All Your Sins Achievement in Halo: The Master Chief Collection

This achievement will unlock once the entirety of the Halo: Reach Campaign has been successfully completed on either a solo or cooperative Legendary playthrough. It is recommended that you play through the game's Legendary Campaign playlist so that this playthrough will also contribute toward the "Who Needs Red Flag?" achievement.

There are multiple methods in tackling the missions of the Campaign, but keep these general tips in mind when playing:

• Utilize the Noob-Combo (Plasma Pistol/Magnum Pistol or Plasma Pistol/DMR) at all times when facing Elites; an overcharged Plasma Pistol shot will drain their shields, and a quick Magnum/DMR round to the cranium will down the warriors easily
• Utilize ground vehicles—Warthogs, Scorpions, and Ghosts—as frequently as possible; their firepower in missions such as ONI: Sword Base, Tip of the Spear, and The Package will help clear out areas quickly and you can even drive past enemy encounters if the mission allows for it (which is more often than you'd think)
• When facing Hunters, get close enough so that they charge at you, allowing time to side step their attack; when the orange flesh on their back is exposed, shoot it, as this is their only weak spot
• When squaring off against Drones, specifically in Pillar of Autumn, keep a DMR or Needle Rifle on hand; these are the best weapons to use against these agile flyers
• Don’t neglect the basics of using cover; when your shields drop down, hide behind a wall or crouch behind an object before charging back out into battle
• Try exploring the missions and finding alternate routes behind or around the enemies; this can help save time by skipping major firefights on levels such as ONI: Sword Base, Nightfall, Tip of the Spear, Exodus, and Pillar of Autumn

Overall, because seasoned Halo players will be familiar with both level design and enemy behavior, completing the entire Campaign on Legendary shouldn't pose too much of an issue. Once all the missions in Halo: Reach have been completed on the Legendary difficulty setting, this achievement will be yours.