DOOM + DOOM II

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

Boot up "TNT: Evilution" goto Level Select and choose Level 30: Last Call, 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 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 and all Keys

When all the cheats are activated, take your first right and walk through the door, you will enter a room with loads of different coloured torches and platforms. Directly infront of you will be 3 Green Torches in a row, walk onto the platform with the THIRD GREEN TORCH that is behind a Red Torch, turn right and you will be looking at another row of Torches that go Blue, Red, Red. While looking this way just walk straight ahead and through the wall without looking/moving left or right and you will end up in a room with a pillar in the middle, a Red Key Door on the right and a CyberDemon directly infront of you near a Lion(?) Switch. While looking at this Switch, walk through the wall and keep going past the first room until you see a big Baphomet skull on your right. Look at the skull and go forward trough it and keep going you will see the Head of John Romero, shoot this and after a few seconds the level will end and the achievement should pop after a few seconds

I know this is a really weird and convoluted way to do this but i found it easier this way, just make sure when you walk through the first door you dont move left or right and follow the Torches

Game: TNT Evilution
Level: 30 - Last Call

Using console commands with the ` key, type 'god', 'give all', and 'noclip'. Open your map with Tab and fly somewhat southeast until you find the boss area with the giant head on the wall. Behind it is Romero's head. Shoot and kill it to complete the level.

EDIT:
For those on console, type these codes in with a keyboard plugged in::
IDDQD -- Invulnerability
IDCLIP -- Walk through walls
IDKFA -- Full health, ammo, weapons, armor and keys