Halo: The Master Chief Collection - Halo 2 - Don't Make a Girl a Promise - Halo 2: Collect the Cortana toy in Gravemind. This achievement is fairly easy to unlock.
On the mission Gravemind, when you exit the building and are at the first wide open area. Drop down walk up the hill, climb up the rock and jump over the path to another rock where you can reach the wall.
Walk all the way to the end back towards where you came where the toy is located.
Run over and it up and your achievement should pop! Enjoy!