I did it like this: I took my recruited players and took them out to sea and leveled them up to 30 (granted this takes alot of time but well worth it) and sharpened everyones weapons to 7 or 6. Next I put an Earth Rune on One of my Characters and went to the Ruins get to the part where you first encounter the Guard and use only physical attacks if you leveled up the characters up right The Guard should go down in no time.

The second time you meet him use the LV.1 magic on the Earth Rune in the beginning then unleash with a barrage of Rune attacks like Soaring Bolt (Lightning Rune) Eternal Ordeal (ROP) and the creature takes a tumble after a few physical attacks BYE BYE MONSTER!!!!