That fight just means Muichiro is stronger than Sound dood after his soul-finding, memory recovery, gene revival, and using his all to the point he can't move after the fight.

I think it was a struggle the whole time, even if it seemed to have ended as a surprise to the 5th ranker himself. If the demon decided to play it safe and prolong the fight, which is common sense for any demon vs human fight, Muichiro would be dead. The demon's arrogance killed him.