Skulltaker Halo 2: Thunderstorm
From Halopedia, the Halo wiki
Skulltaker Halo 2: Thunderstorm is an achievement in Halo: The Master Chief Collection. It is awarded for collecting the Thunderstorm skull in Halo 2: Anniversary on Cairo Station while playing on Legendary difficulty. The achievement is worth 5 Gamerscore, and the Xbox achievement art is a still from the Halo 2: Anniversary opening cutscene from The Oracle, depicting a Special Operations Sangheili.
In the large room after the armory, you need to use a Crouch Jump to get onto a light fixture on the right wall (right of the room when you enter) and then jump from there onto the higher area. There is a series of diagonal beams. Walk up one near to the end of it and use it to Crouch Jump onto one of the rails. You can fall between the rails, so be careful. Once on the rail, walk down it until you can jump onto the highest platform. Then melee the trashcan away from the wall a few times, the Skull is right behind it.