As mosy people have already stated, Diamond weapon is fairly easy, and almost any level would suffice, as long as you have a correct stratagy.

I recently completed FFVII, I played FFVIII,FFIX,FFX,FFX-2,FFVI then FFVII, in that order. I found FFVII to be one of the quickest, easiest Final Fantasy games, as you can uncover all secret/side quests easily, and still finish the game with a low time score.

I defeated Emerald and Ruby weapon at level 60, so I doubt leveling to 99 is worth the time ^_^