DOOM + DOOM II

if you are doing this on Xbox you will need to plug in a keyboard to type in cheats

Boot up Doom 2 and start the first level on any difficulty (I left it on "Hurt Me Plenty") as we will be using the God Mode cheat

When you spawn in, type "IDCLEV30" and it will put you into the final level
Type in "IDDQD" for God Mod
Type in "IDCLIP" to be able to clip (walk) through walls
You can type in "IDKFA" to get max ammo for weapons, but not needed

With the cheats activated, keep walking straight through the wall (the scrolling one with the faces) and you will see the Icon of Sin (Baphomet) with a Red hole in the middle of its head. Keep walking and drop down, continue walking forward into the wall and you will Clip ontop of it and be face to face with the Icon of Sin. Walk into it and you will appear in a corridor the the Head of John Romero, i shot it once with the Double Shotgun and after a few seconds the level ended and you should get the achievement not long after

If (like me) you have an Xbox but no keyboard, you can just select the level in the level select options. You don't need to play through any other level of Doom II. Only the very last one counts for the achievement.

Single Player > Level Select > Map30: Icon of Sin
Difficulty: I'm Too Young To Die

Grab all the weapons if you want but you'll only need the rocket launcher for this. Enter the teleport and then make your way to the top of the "structure" by interacting with the demon symbols to lower the walls. Interact with the electronic skull to raise the pillar in the lava.

Once the pillar is fully raised, jump next to it and interact with it to lower it down, then grab an invincibility orb and as the pillar is rising shoot a rocket into the "brain" area of the demon skull on the wall. 3 Rockets and it'll blow up!

If you're having trouble getting a rocket in, shoot as the cross-hair is in-line with it's eyes.

Achievement earned!

Plug in a keyboard to your Xbox.

Start doom ii so you load into the first level.

Type in IDCLEV30

This will warp you to level 30. Then type IDDQD to go invincible, then type IDCLIP to go through walls. You can then go behind the icon of sin, kill John Romero's head, and get this achievement.

This works as of 8/8/24. I expect this to be patched.