Kingdom HeartsIn Kindgom Hearts, one can attempt the amazing feat of defeating Sephiroth in true combat. Sephiroth has unbelievable skill, and is difficult to defeat for most Kingdom Hearts players. Typically players between level 70 and 89 have trouble beating him, but he becomes easier upon reaching the 90s. Having maxed out stats using stat boosts is also helpful.
Kingdom Hearts IIn Kingdom Hearts I, you can battle Sephiroth after beating the Hades Cup and selecting the second "?????" option, which is the Platinum match in which you battle against Sephiroth. Sora fights this battle alone, without Donald and Goofy. Be sure Sora is at least level 65 or above, otherwise the battle will be even more insanely difficult. Aero, Cure, Elixirs, and Ethers will be your most used commands throughout this battle. Attack with one combo when you can get close enough, then immediately retreat to avoid a counterattack.
Kingdom Hearts IISephiroth appears just beyond the crystal fissure in the very last section of Hallow Bastion, only after you seal the keyhole in Space Paranoids. To defeat Sephiroth, you will likely need Ultima Weapon, plenty of elixirs, and a lot of skill.
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