Defeating the demon lord's a cinch (if you've got a ringer). volume 1 - Tsukikage

Ten years have passed since the Demon Lord Kranos declared war on mankind. As the situation grows desperate, the Ruxe Kingdom summons a hero from another world to stand against this threat. Assigned by the Church to join the hero's party, a battle-worn priest arrives to fulfill his obligations but is shocked to find that his companions are utterly inept! A mage who can only use fire magic, a swordswoman recently defected from her father's fighting school, and a hero obsessed with women?! With an average level totaling just 15, there's no way these novices are capable of taking down a demon lord! But a job's a job, so it's up to the man of God (who thankfully has power to spare) to support these buffoons from the shadows. With an extraordinary healer on their side, defeating their target will be a cinch! ...Hopefully.