When facing Abaddon, equip Death as your Guardian Soul. Death's scythes will repeatedly attack Abaddon for about 30 to 35 HP of damage per hit. Death's Soul also will destroy some of the locust swarms that Abaddon uses. Keep evading Abaddon and the locust swarms, and use Mind Ups or Mana Prisms to keep your MP filled. Use potions or food when your HP gets low. When using a combination of physical attacks and Death's Guardian Soul, it will not take long to take down Abaddon.