Wow.. That's awesome, and I know that The Doom Slayer is powerful and strong enough to contend with Oni. It's due to his Super Strength and Speed given to him by the Seraphim sometime after losing his homeland, brother's in arms, and people. In the real world, that would make him powerful enough to kill any demon, and if they regenerate, he could absorb their soul. It's mentioned somewhere in the lore that he can do this, hence why he gains Health, Armor, and Ammo from those he kills.