Chainsaw Man

[Buffalobiian]

Btw what WAS that demon's weakness now? Felt like Denji just kept slashing away at the devil until the heart was exposed.

Denji didn't get to his heart. The Infinite Devil presented it to him.

"Stop cutting me endlessly and end this. Please." -Effectively.

His weakness is that he is psychologically weaker than his infinite body, so if someone with a stronger (or crazier) mind attacks him with a body that can keep up with infinite regeneration, the devil succumbs.

The demon will endlessly give blood, and I'm sure it's not just Chainsaw who would benefit from that. Power for example, would also benefit. But whereas most other people would go "There's no end to this", Denji goes "There's no end to me!"