Things You'll Need
Instructions
Choose your main characters. There are three characters that you can have in your party. It doesn't matter who you pick, but make sure Yuna is one of the three. It's not required, but Yuna should have the optional aeons.
Equip Auto-Phoenix Armor, Celestial Weapons and Auto-Haste to the three main characters. Also have them learn Quick Hit. Use Quick Hit in battle against Nemesis.
Watch Nemesis attack. Have Yuna start summoning before Nemesis' turn when he uses the attack Ethereal Cannon. Don't use the aeons' overdrives because Nemesis will counter with his Ultima attack, Armageddon, which does 99.999 damage. Keep summoning until Nemesis uses an attack called Ultra Spark, then you don't need to summon anymore. Yuna can summon until Nemesis uses his Armageddon attack. You should be able to beat Nemesis before you run out of aeons if Yuna has all the optional ones.