Toughest bosses in disc 4 (from toughest to lamest)

1. Ozma
2. Deathguise
3. Nova Dragon
4. Tiamat
5. Maliris
6. Kraken
7. Hades
8. Kuja
9. Lich
10. Necron

But it really depends how tough you are as well i wasn't that strong yet when i was gonna enter Memoria and i encountered that Nova Dragon, so....

And Necron is too easy no doubt about that!