DOOM + DOOM II

(Plug a keyboard into your xbox in order to use commands)
DO this on E1M1: Hanger set it to ultra violence and turn on the Invulnerability cheat IDDQD you need 100% kills 100% items (health packs dont count) and 100% secrets.

Do not use IDKFA as it will mess up the 100% counter you have to be exactly at 100% for items secrets and kills.

if you cant pick up anymore ammo use the gun related to that ammo to make more room

just make sure you pick up the blue armor vest last or you wont be able to pick up any more armor.

Heres a video if you dont know where all the secrets in this level are located

(Plug a keyboard into your xbox in order to use commands)
DO this on E1M1: Hanger set it to ultra violence and turn on the Invulnerability cheat IDDQD you need 100% kills 100% items (health packs dont count) and 100% secrets.

Do not use IDKFA as it will mess up the 100% counter you have to be exactly at 100% for items secrets and kills.

if you cant pick up anymore ammo use the gun related to that ammo to make more room

just make sure you pick up the blue armor vest last or you wont be able to pick up any more armor.

Heres a video if you dont know where all the secrets in this level are located

I'm sure the other solutions work fine, but it will probably take you longer to read this guide than it does to actually get the achievement with this method... if you have a keyboard.

Choose Doom II, and select Level 30 Icon of Sin from the Level Select options (or you can start a New Game and input IDCLEV30 on your keyboard to essentially warp there from the beginning). Make sure you choose Ultra-Violence difficulty before starting the game.

Once you're in the intended level, type IDDQD on your keyboard and grab the rocket launcher amongst the weapons in the starting room (or just type IDFA to instantly obtain all weapons), then type IDCLIP. Run straight through the wall where the gray teleporter is, and keep running towards the evil looking face in the wall in the distance. Once you walk through that face in the wall, you'll want to stop immediately, as you'll see a man's face (John Romero, if you're wondering). Simply select your rocket launcher (option 5 on keyboard), and shoot John's face.

The level will end shortly after, and you'll be credited with 100% kills.

Since you had IDDQD active, you didn't take any damage either.

If you hadn't beaten Doom II yet, you'll also get the achievement for doing so.

If you're unclear on anything, please let me know.

Weirdly, this unlocked for me on E2M8 while trying to get the achievement for killing a Cyberdemon with the pistol. I saved repeatedly while wearing it down, then accidentally killed it with a rocket and finished the level. I reloaded my save and tried again after skipping the cutscenes back to the menu (Edit: this seems to be what triggers the bug. Both IchFTW and Azathoth1488 seem to have confirmed it in the comments.), then accidentally killed it again after reloading. This time, however, I didn't take damage from the point where I loaded in to when I killed the Cyberdemon, and this unlocked.

(Fun fact: Lost Souls don't count towards 100% kills. They haven't since very early builds of the game due to bugs that caused Pain Elementals to sometimes generate them out of bounds in Doom 2. They retroactively patched the first game to change that.)

I found that DOOM 2 level 32 was a great place to get this achievement. Here is how I did it

I found that DOOM 2 level 32 was a great place to get this achievement. Here is how I did it

DO this on E1M1: Hanger set it to ultra violence and turn on the Invulnerability cheat IDDQD you need 100% kills 100% items (health packs dont count) and 100% secrets.

just make sure you pick up the blue armor vest last or you wont be able to pick up any more armor

Heres a video if you dont know where all the secrets in this level are located

Just so know you need everything 100% for this achievement not just kills.