First and foremost you must have agility, magic, strength, defense, and magic defense at 200+ First off cast hastega, protect, and auto-life,(don't worry about casting shell). When attacking Nemisis always use Quick Hit. Attack with your party untill Nemisis uses its cannon on you after he uses the cannon he will attack with either Armaggedon (which causes 99,999 damage to all characters) or his buster attack (which causes 30,000 damage to all characters). Summon an aeon right after he uses the cannon attack against you, but be aware as sometimes Nemisis can use one of those two powerful attacks without warning at anytime that is why auto-life is needed. Unless his first attack after the cannon is Armaggedon after sacreficing an aeon return to your normal attack pattern using Quick Hit. If he uses Armaggedon then summon another aeon before he attacks again as he will use the Buster attack. Nemisis shouldn't take to long to beat just keep using Quick Hit and time his attacks correctly. Stay focused because one mistake in this battle can cost you dearly. Also don't use overdrive attacks against Nemisis he will counter with Ultima. If you want to steal an item from him use mug because he will also counter a normal steal attack.