Mean Streets of New Mombasa Achievement in Halo: The Master Chief Collection

This achievement will unlock once Mombasa Streets, all eight missions, and the cut scenes (Prepare to Drop and Epilogue) of the Halo 3: ODST Campaign have 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 "Trifecta" achievement. 

Take note that "Mombasa Streets" is treated as one level, with each visit set as a different Rally Point. This means that after completing the first street segment (before Tayari Plaza), you can skip all of the following visits by quitting the Campaign and selecting the next mission through the Campaign lobby menu, provided you aren't playing as part of a playlist. If you do this you will have to return to these points later on for all the clue related achievements, though you can do so on Easy rather than Legendary.

Unlike the other games in the series, most of the missions in this campaign are very linear or “defense based,” and simply don’t have the “out of map” or “rushing” tactics some players may be familiar with from Halo 2 or Halo 4. Despite this, keep these general tips in mind when playing:

• Utilize the Noob-Combo (Plasma Pistol/Pistol) at all times when facing Brutes; an overcharged Plasma Pistol shot will drain their shields, and a quick Pistol round to the head will guarantee a kill
• In Uplift Reserve, utilize the Covenant Ghosts to quickly pass through the entirety of the mission as if you were speed running, thus lessening the amount of killing needed and overall time spent on the mission
• When squaring off against Drones in Data Hive, keep a Covenant Carbine or Beam Rifle on hand; it’s the best weapon to use against these pesky, 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
• Don’t hesitate in giving your power weapons (Rocket Launcher, Fuel Rod Cannon, etc.) to mission-essential NPCs (such as Mickey during ONI Alpha Site or Buck during NMPD HQ and Coastal Highway) as they are invincible and have unlimited ammo

Overall, because seasoned Halo players will be familiar with both level design and enemy behavior, completing the entire Campaign on Legendary will be quite easy. Once all the missions in Halo 3: ODST have been completed on the Legendary difficulty setting, this achievement will be yours.