Yikes, I'm at the same part Risha. (Playing through X-2 again wasn't as fun as I thought it would be) Basically, if you have a Dark Knight that has 5000 or more HP, and knows the ability Charon, you could kill him in one hit.

Though I haven't tried this, it should work like a charm. (I'll try it tomorrow to let ya know.)

There many other ways to kill him, which I don't know of.