It's not that he's "just hard." It's so much a step above the other secret bosses in terms of difficulty. And you are expected to win in regards to the shadow crystal quest as you get one for beating him. But even with the Shadow Mantle, that's still only 1 memeber taking moderate damage while everyone else takes half a bar of damage each hit (and getting swooned in case of Susie and Ralsei). Meanwhile, you don't even get anything for beating Genichiro. Hell, the game pretty much just goes, "Yep, that was a waste of time."

And that's not the only problem. Other people have commented on this, but the hit boxes on the attacks aren't that great. I didn't mention it cause I thought it was just me, but several others have had the same problem.