Sword and Shield
From Halopedia, the Halo wiki
Sword and Shield is an achievement in Halo: The Master Chief Collection. It is awarded for activating the seventh Halo 3 Terminal on Halo. The achievement is worth 5 Gamerscore and can be unlocked on any difficulty. The player must scroll through and access both parts of the terminal to unlock the achievement.
The final terminal is found at the beginning of the leve. The player must head forward from the start of the level through the snow until they encounter the area with a few structural beams going from one wall into another. At this point, before going past any beams, the player will see a metal floor. Following the floor to the right will lead the player up to a beam heading through a crack in the ice. Follow this beam into a doorway. Cortana will ask, "Where are you going?". Turn left once inside and the terminal should be in full view. Cortana will say, "Wait. What's that?" as a hint. The terminal will be in front of what is presumably one of Installation 04 (II)'s pulse generators.