Skulltaker Halo 2: IWHBYD
From Halopedia, the Halo wiki
Skulltaker Halo 2: IWHBYD is an achievement in Halo: The Master Chief Collection. It is awarded for collecting the I Would Have Been Your Daddy skull in Halo 2: Anniversary on Outskirts while playing on Legendary difficulty. The achievement is worth 5 Gamerscore, and the Xbox achievement art is a still of Tartarus from the Halo 2: Anniversary opening cutscene from Cairo Station.
The skull has a 1 in 7 chance of spawning every time you play the level. There is no connection to any number of checkpoints gained or enemies killed. The only checkpoint you must trigger is the one inside the skull room. The only triggers, beyond the 1 in 7 odds, are playing on Legendary solo (the skull won't spawn in cooperative play) and spawning the Jackal snipers in Sniper Alley.
You can progress through the level normally to get IWHBYD, but the easiest way is as follows. From the beginning of Outskirts, run into the first hallway. A single Grunt will confront you. Kill it, and then look overhead. You will see two lights on either side of the hallway. Jump on to one of them, and then crouch jump up to the higher platform. From there turn around and run along the thin ledge to the right of the alleyway below.
Across from it, you will see another platform. Jump to it, then Grenade Jump or crouch jump onto yet another higher level. From there, look left and you will see a nearby rooftop, jump to it; go to the right and head down that alley, where you'll be able to jump onto the higher rooftop. Head across the rooftop then drop down on top of the broken highway.
Grenade Jump or jump on the banister and crouch jump onto the rooftop ahead and slowly move towards the opposite edge of the roof until the Jackals spawn in. When they do, immediately move back from the edge and look to the left. Backtrack to where you first jumped onto the rooftop in front of the bridge and head left towards where the Pelican leaves. Run along the rooftops until you reach the flat part of the bridge and jump down. Jump from the bridge into the alley below and run up the cement ramp. Turn right and you'll see some boxes. Jump onto them and get onto the roof to your left. From there, look right and jump onto that rooftop. Head to your left.
You should now be on the other side of the sniper alley, facing a small bridge where a Jackal Sniper spawns. Head to the right and you'll see two interconnecting balconies. Head to the one furthest from you, and go through the door. If you're lucky you'll see the Skull between a sniper and some ammo (wait a couple seconds because it can take a little bit to pop up). If the skull is not there, make sure you have hit the checkpoint inside of the room by crouching by the wall the skull is in front of, or restart the level (reverting to the last checkpoint will not work).
The achievement will unlock as soon as you grab the Skull, upon which you'll be transported into the alley. You will have only a Plasma Pistol, and you will face seven waves of Ultra Elites (wave one starting with one Elite, wave two with two Elites, and so on). Unlike the original game, defeating them is not a requirement.