The thing is, you can beat Sephiroth on level 50-60. And Meteorain will do the job. You don't need KOTR...

Although, seeing as you do have level 4 limits and KOTR and ultimate weapons...Sephi's gonna be a breeze if you went in now.

But, if you are one of the people that demand a perfect game...I spent most of my time before the Northern Crater training around Mideel. I'm not sure if there's any better places.