Sephiroth takes some getting used to. The best strategy is to use only Strike Raid for keyblade abilities because you are invincible while using it. (unequip sonic blade, ars arcanum, etc)
Basically, Seph has 3 stages. The first is him slashing at you, teleporting and slashing at you, then occasionally using nova. When he uses nova, dodge it then Strike Raid him, cuz it's a lot harder to miss if you use it then. Once his health bar starts moving, he moves to stage 2. Which is the same as stage 1, except that he will randomly teleport to the corner of the arena and use Sin Harvest, which takes away all of your mana and all but one hp, regardless of how much you have. The only way to block Sin Harvest is to hit Sephiroth before a little rainbow appears above your head. You basically have to practice locking on to him and hitting him with Strike Raid fast enough. (he says Sin Harvest! before he actually hits you with it)
Part 3 is easier than part 2... I'd recommend being at least level 80 and having the Ultimate Weapon... Though that's just me. Hades Cup should be a cakewalk if you're that powerful. Ansem is pretty easy as low as level 60 or thereabouts, much lower than that and some effort is required.