DOOM + DOOM II

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

Boot up "The Plutonia Experiment" go to Level Select and choose level 30: "The Gateway of Hell" difficulty doesnt matter as we will be using the God Mode cheat (i left it on Hurt Me Plenty)

When you spawn in start typing in the cheats:

Type in "IDDQD" for God Mode
Type in "IDCLIP" to be able to Clip (walk) through walls
Type in IDKFA" for max ammo for all weapons

When you start you will see a MegaSphere directly in front of you (whiteish orb with a face on it) While standing on this platform turn 90 Degrees to the left looking at the 4 Ammo Boxes, walk through this wall and keep walking forward a few rooms until you see the giant skull of Baphomet (has a Red Square in the middle of its skull) Walk into this skull and you will clip into a corridor that has the Head of John Romero, shoo this with whatever weapon you want and after a while you will end the level and the achievement should pop up not long after

Start a new game in Plutonia

Type IDCLEV30

IDDQD god mode, IDKFA for full inventory, and IDCLIP so you can head straight north through the walls until you reach the Icon of Sin (giant goat skull, clip into its forehead and kill the Romero head)