Jeanny, her battle dragon, her spirit weapon, Honuren, and Eoranth fought Satan. Satan's HP was around one-third. Under normal circumstances, it should be possible to use burst attacks by linking multiple highly damaging combos to drain this remaining HP away, just like Jack did at the beginning of the duel. However, since they started fighting, none of them had managed to land an attack on Satan.
The devil just dodged their attacks while giving them a mocking grin. Attacks he couldn't dodge were blocked by the Soul and Chaos Rings of Defense. His HP was slowly recovering. His final form didn't improve his natural HP recovery. But if he didn't suffer any hits, it was the same as he kept recovering.
Satan finally received a hit when Paytowin joined the battle and shot using his rifle from far away. The damage was minuscule, though. Satan's final form enhanced his defense, which was already boosted by his low HP.