Halo: The Master Chief Collection - Halo 2 - The Will of the Prophets - Halo 2: Collect the Arbiter toy in Sacred Icon. This achievement is fairly easy to unlock.
On the mission Sacred Icon (Use the sputnik skull), as you cross the first bridge take a left at the end. There will be a short pillar you need to perform a grenade jump on to reach the area above.
Once you make it up, you need to proceed all the way down and on the left there is a dark area you need to jump in where the toy is located.
Run over and it up and your achievement should pop! Enjoy!