Stat me

...

Large Magical Beast
Hit Dice: 5d10+25 (52 hp)
Initiative: +1
Speed: 30 ft. (6 squares)
Armor Class: 15 (–1 size, +1 Dex, +5 natural), touch 10,
flat-footed 14
Base Attack/Grapple: +5/+14
Attack: Claw +9 melee (1d6+5)
Full Attack: 2 claws +9 melee
(1d6+5) and bite +4 melee
(1d8+2)
Space/Reach: 10 ft./5 ft.
Special Attacks: Improved grab
Special Qualities: Scent
Saves: Fort +9, Ref +5, Will +2
Abilities: Str 21, Dex 12, Con
21, Int 2, Wis 12, Cha 10
Skills: Listen +8, Spot +8
Feats: Alertness, Track
Environment: Temperate
forests
Organization: Solitary, pair,
or pack (3–8)
Challenge Rating: 4
Treasure: None
Alignment: Always neutral
Advancement: 6–8 HD
(Large); 9–15 HD (Huge)

I want a DOOM RPG. What's a good system for running a game based on DOOM?

A personal home computing device

He does seem like a big guy.

Simply ebin

In MAID, because why not
Monster/Shadow
Root: Hatred
Green Suit
Sunglasses, Quiet

Athletics 8
Affection 2
Skill 1
Cunning 3
Will 12
Luck 0

Powers: Surprise Attack, Giant Weapon
Weapons: Bazooka & Machinegun

for you.

How do you stat exponential and infinite demon killing?

There is a Doom RPG. It's called such.