I agreeBeing able to fight something insanely powerful and supernatural doesn’t require one to be supernatural.
BUT
If you aren't supernatural, the game either requires additional rules or does not use real world physics or biologyThat's the core issue.
"I want Real Earth mundane martial to level 20" and "I don't want additional rules" cannot exist together.
Being 20 feet tall.or just a human fighter. What’s the devil doing that requires anything more?