Black Mages make Orthros easy. I had a much bigger problem with the next crystal boss than him, until I realized that
his physical attack is also water-based, so you can use water defense skills to lower the damage you take.
Also, friendly warning: There's a spoiler in the artbook just by placement of one of the character's art in a certain section. I'd recommend not going more than, say, half in.
On an unrelated and pointless note, I love the Hartschild theme. It's really the first noteworthy song on the soundtrack for me.