Yes, he is. He's probably the 3th or 4th hardest boss so far even at the point where I am. Exor is just being a plain hell, though. Every time Exor's mouth attacks, there's a 50% probability that you'll cry.

I'll try Exor again soon though. I'll equip all the most attack boosting equipment I have on Geno and try to beat Boomer without losing any important Items.