DOOM + DOOM II

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 Master Levels. Only the very last one counts for the achievement.

Single Player > Level Select > Level 20: Mephisto's Mausoleum
Difficulty: I'm Too Young To Die

Initially this looks like a tricky level but it's actually quite easy. The level starts out with 3 Revenants walking about. Grab your weapon of choice (I used the shotgun) and then take them out either by circle strafing (if you're good at dodging) or by rushing them and then hiding behind a wall to avoid their homing missiles. Keep in mind that the chaingun can to a certain extent stunlock them but I prefer the shotgun.

When they're down look for the discoloured pillar and interact with it. If you can't find it, it's the one in-front of the locked blue door. This will reveal a teleporter and another Revenant. When it's safe to do so, hop into the teleporter and be ready for a fight!

If you lose too much health in the initial fight, the beserk pack will heal you back up to 100%.

Turn around, grab the invincibility and take out the 2 Revenants. Now the tricky bit. Each "window" above an icon of sin will have a Mancubus in it. Grab the plasma, stand in the middle and shuffle back and forth while firing and hope that'll lock onto them. If you run out of ammo, each of the pits has a couple of backbacks of ammo and there's a few health packs dotted about for healing.

Once they're dead, the walls will come down in the 4 outer pits and 4 Hell Knights will enter the fray (don't worry about the Arachnotrons). Kill these and then grab the red skull key which is in the NW corner and the red door is in the SE corner.

Back outside, kill the 8 Revenants that spawn in, open the red door in the NW corner, enter the building and as you get close to the switch a Revenant will rise from the tomb. Kill it and then click the switch.

This has lowered the walls in the SW corner of the building allowing access to the mega armor. Grab it and then head up the steps which have just raised up. Grab the supercharge and then enter the room with the arrow on the floor. Run through back into the arena, grab the blue skull key and then exit out through the SE corner back outside again. Nearly there!

Open the blue door, kill the 2 Barons of Hell, grab the megasphere and then save the game. The next bit is tricky.

Return to the small building in the NW corner and you'll notice that the edge of the tomb has become a lift. Stand on the tomb, hop over the gap into the alcove where the stimpack is (to the right of the switch), open the wall on the left and grab the invincibility.

Run back to the room where the Barons of Hell were, ride the lift up to the top, point your rocket at the icon of sin opposite and unleash rockets until the level is done.

Achievement earned!

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

Boot up "Master Levels for Doom 2" goto Level Select and choose Level 20 "Mephisto's Mausoleum" and choose whatever difficulty you want as we will be using the God Mode cheat (i left it on Hurt Me Plenty)

Type in "IDDQD" straight away for God Mode as there are multiple Revenants roaming around who will fire at you as soon as you spawn (at least on Hurt Me Plenty not sure about lower difficulties)

Type in "IDKFA" for max ammo for all weapons
Type in "IDCLIP" to be able to clip (walk) through walls

When the cheats are activated, walk directly into the walls of the building infront of you and keep going straight until you land in a room with 4 giant skulls of Baphomet. Stand in the middle platform and turn right until you see a skull that has what looks like a pile of skulls and skulls on pikes on either side of it (they have an orangey/firey glow) Walk into the skull and you should see a Mancubus (fat thing enemy) keep walking forward into another room that has red/blue glowing walls and is full of Invisible Pinkies, use any weapon you want to kill them (i just used the BFG to clear them quickly) and then shoot at the Head of John Romero to complete the level

Shouldnt be too long for the achievement to pop on the Stats screen

Select the Master Levels for Doom II game, Single Player, Level Select. Then Choose Level 20: Mephisto's Mausoleum, and you can put it on the easiest difficulty "I'm too young to die."

Start the game, and right away use the Invincibility code by typing IDDQD on your keyboard or chatpad. Then type IDKFA for all weapons. And then type IDCLIP to be able to walk through walls.

Walk forward into the building in front of you until you are in the room with 4 giant horned skulls, 1 on each wall. Push Y to open the map so you can know which way to face, point towards the Eastern wall on the right. Walk forward through the skull wall, then forward through the next wall into the room with the severed head, simply blast the head with your strongest weapons until you kill it to complete the episode and unlock the achievement.