really i found him pretty easy
here i found this on the internet
If you are in Disc II, then all you have to do are summons. But say you are in Disc I. Then what? Perhaps you've built Yuffie up to her Level 2 limits, or Level 3. In that case, just use Bloodfest 3 times while keeping her health up. Other than that...

Move to back row. Now, cast Barrier, MBarrier, Regen, and Haste. Start out your damages with Demi or Demi2. After that starts to do about 2000 damage, use summon Kjata (you can go to the Ancient Forest first, get it, come back and fight) Bahamut, and Odin. Heal when necessary. However, if you run out of summons, don't bother casting Demi, he will keep healing himself. Instead, just cast Demi1, and have him waste 24 MP on curing himself. When he runs out of MP, assault! For winning you get Summon Leviathan and Yuffie's L4 Limit manual.