Idiot: Well, beating Emerald Weapon its not that hard (tho its not easy also lol) as long as you have good level, can summon Knight of The Round, and have Final Attack materia linked with Phoenix
For me its Ultimecia from Final Fantasy VIII. Darn, she beat me up on my first play

and i have to wait 6 years to pay her back coz i dont have a change to play it again
I didn't have all the things I needed when I first took on Emerald which is why it took me a while. I did beat him without the underwater materia or the final attack materia though, once even without knights of round as I just used mime with omnislash as much as possible and Yuffie's limit break for emergency healing. Ultimecia can be overpowered with limit breaks too if you collect lots of aura spells (you can draw them from Seifer in the last battle with him). Hero drinks make it easy too. The hardest thing with her was having to divide up your stuff between 6 characters instead of 3 cos you didn't know which ones she'd select.