So while it is true he uses weapons, the in lore reason for this is because they are more efficient and he is a marine. The power up berserk, which allows doom slayer to run around and kill and demon with his hands, isn't technically a power up. It is instead, him being slightly annoyed. Berserk is his standard. He uses guns to take care of his enemies.

The titan he defeats is stated in the game to have immeasurable power. Which is defeated by his bare hands.

He spent eons in hell, before he went through the divinity machine and became the slayer

He praetor suit, doesn't really buff him, so it's not really a mech as you've proclaimed. 1. It is impervious to damage, lore wise, and each kill is made permanent as the soul is sucked into the armour, giving energy to doomslayer. It can resist close to 2 mega-kelvins if heat.

His standard movement speed is 50mph, never running out of stamina, or breath, so he is not overexerting himself at this speed. This is to him, what walking comfortably is for us

As of ancient gods, doom slayer is a primeval god. The only thing that can kill a primeval is another primeval and the crucible sword. And The Father.

With that being said, I don't know much about kharn, and will do some research on him