Halo: The Master Chief Collection - Halo 2 - Reviewer - Halo 2: Activate Terminal 12 on High Charity. This achievement is fairly easy to unlock.
On the mission High Charity, midway through the mission you will take an elevator up into a room where you will have to fight flood and covenant.
After you defeat them all a large tall door will open up. Go through the door and turn to your left where the terminal will be up against the wall.
Run over and interact with it and your achievement should pop! Enjoy!