In the Eternity Devil arc, Special Division 4 is sent into a hotel to kill the Eternity Devil who is said to reside in an hotel. After realizing that they are trapped in there, the team falls into disarray. The devil offers to release them if they feed Denji to him. Kobeni attempts to kill Denji to accept the Eternity Devil’s offer, but fails. Finally, the devil was defeated when Denji jumping straight into its mouth, sawing through its flesh, finding its heart and destroying it.
spoiler
Um actually … Denji did not find the devil’s heart. The devil’s heart was inaccessible to the protagonists, and thus it was effectively invincible. However, it still can feel pain and thus was unable to bear the torment of Denji repeatedly attacking him for days, and ultimately revealed the heart to Denji to destroy.