Instructions
Start the battle off by having Sazh cast Haste, Bravery and Faith on both herself and Vanille. These buffs will help increase their speed and attack damage.
Have Vanille, while Sazh is casting buff spells, cast Imperil, Deprotect and Deshell on the Reaper. Casting these debuffs will make the Reaper take more magic, physical and elemental damage.
Remove the Nerve Gas debuff the Reaper uses with War and Peace, as well as the medical spell removing poison.
Use the abilities Slash &Burn and Dualcasting to damage the Reaper. When the Reaper does attack, he does a large amount of damage, so be ready to use one of the characters for healing items or casting a healing spell.
Continue to use damaging attacks, or fire and ice spells, to defeat the Reaper.