Halo: The Master Chief Collection - Halo 2 - Pyrrhic Solution - Halo 2: Collect the Didact toy in The Great Journey. This achievement is fairly easy to unlock.
On the mission The Great Journey, in the beginning of the mission you need to to take the revenant with you. Drive it to the end of the path and park it under the overhang and get out to proceed into the building.
After you exit the building and trigger the cutscene, get in a banshee and fly down and get into the revenant. Drive towards the mission objective where the scarab destroys the door and drive into the water.
Turn it to classic graphics and go to the right side and drive up the hills. The toy is located on the tallest hill.
Run over and it up and your achievement should pop! Enjoy!