Legend Slayer Achievement in Halo: The Master Chief Collection

In regards to the Legendary Anniversary achievement, this is awarded to players who have completed the Halo 2 Campaign on the Legendary difficulty. It's hard to gauge whether this achievement will be common among the online community, but since Halo 2’s Campaign is at the forefront of this collection, it should be quite common in due time. Thus, continue to spend time in the Multiplayer and this achievement will be yours in no time. Alternatively, you can also boost this in a Custom Game if you have a friend who has obtained the Legend Anniversary achievement.