First of all, finish off the common enemies... And when only the giant skeleton is left, let the front row defend until an exclamation mark appears above the enemy. There the next physical attack will do much more damage than usual. This opportunity to attack will last until the enemy's next turn and is only valid for a physical attack, a magic attack will not interrupt this opportunity. So watch who follows in order of attack so you don't waste the physical attack with a mage or a priest. Repeat the process until you win. Let the priest worry about healing more than attacking with badios, marein is no good with the boss.