I wanted to play DOOM as an RPG. (Happy 25th anniversary, DOOM!) There are two things you need:
Stay Frosty and
Exemplars and Eidolons.
Stay Frosty is an RPG about playing a bunch of Doom Marines or Aliens' Space Marines running around, getting stressed, which makes you more competent until you make an unlucky save and then TENSION EXPLODES and you're murdering everything and ignoring tactics, possibly until you die. It's like if Mothership were about just playing marines and people panic by doing Billy-style last stands from Predator. Remove armor, throw away gun, attack big bad with a giant knife. The reason I don't just use it vanilla, is that the PCs are still fairly flimsy, _especially_ if you want to just run 1 PC against squads of enemies like a Doom Slayer.
To up the PC power level, I use damage, Gifts, and HP/HD from Exemplars and Eidolons.
Exemplars and Eidolons is about playing supernaturally-powerful thieves, wizards, or warriors. Your warrior can take on groups of 1 HD fools and destroy them all, punch through walls, never needs to sleep, that kind of thing. E&E evolved into Godbound, where the PCs are even more over-the-top, but E&E is perfect for what I want to play: solo Doom Slayer vs the first level of that game.
So your PC has 4 stats (Stay Frosty): Brawn, Brains, Dexterity, Willpower. Roll 3d6 and sum. You roll _equal to or over_ your stat: So Brains 9 is like '9+', and you want to roll a 9 or higher. If none of your stats are 5+, set one to 5+. Enemies try to hurt you (or terrify you) by rolling a d20 under your stat, and you subtract the difference between their level and yours from the roll.
Stay Frosty is a Black Hack hack that figured out to make everything the PCs want to do roll over. 20s are good! You use your stats to resolve attacks, unbreaking an APC's engine, wire electricity through a bulkhead, dodge falling rains of fire, that kind of thing.
Tension is as per Stay Frosty - you get tenser and better at kicking ass, until Tension Explodes and you flub your Willpower save and then you're fighting a cyberdemon with a knife. There's also critical hit and critical fail tables in there that I'll use. And enemies, mission generators, pointcrawling advice.
You can carry 21 - Brawn items.
You have 8 HP and gain 4 per level. Monsters use their HD as their HP. Armor is ablative HP that gets ruined and replaced just like in DOOM. Back-and-breast armor has 2 hp, a helmet has 1 hp. You could probably cobble together arm and leg armor and have up to 5 hp - for a 1 HD soldier that's a lot of extra survivability. Perhaps too much, we'll see when I run this more.
Damage is: weapons do 1d6 to 1d20 damage, a roll of 1 is 0 damage, 2-5 is 1 hp/HD of damage, 6-8 is 2 hp/HD, 9+ is 4 hp/HD. A combat knife does 1d6 damage, a pistol does 1d8, as does a 1-handed sword, as do some cheap assault rifles. A good rifle or two-handed weapon does 1d10 or more damage - a chainsaw might do 1d12, as might a super shotgun at optimal range. A BFG does 1d20 damage to an area of enemies but has like 1-2 shots taking up inventory slots.
So the damage is one huge thing that ups a single PC's survivability. You start with 8 hit points, you can take out 1 HD enemies with 1 hit, you can probably go toe-to-toe with a decent-sized demon/alien monster and survive.
You get 2 Effort, from Exemplars and Eidolons. You spend that to use some of your E&E Gifts.
You get some of the Warrior or Generic gifts - 3 at level 1. So you might be able to fight large groups of your-HD-or-less monsters and only get attacked once, or you can rip bulkhead doors down given a few minutes, or you can spend effort for the combat to nat 20 hit your enemy. To me this is another thing that we see the Doom Slayer do all the time.
