From The Abyss Cheats

Use the following tactics to make the final Boss battle (Abyss 8 - Floor 4) easier. The recommended level is 55 or more. Have a very good spear and good bow. The first part of the Boss will be a giant cocoon. He is not very difficult, just very annoying. Equip the bow and have the skill Fine Moment set. Walk up to the cocoon, and use Fine Moment eight times (or until it will not allow you to cast it again), and use Invisible Shot until it is about three-quarters dead. Then, press A, and it should die. You will see an intermission sequence of the world being absorbed into darkness. Once the darkness clears, you will see the final Boss (The Abyss King). There are a couple ways to kill him easily:

1. Equip the spear, and have the skills Gallop Lancer, Thousand Spear, and Poison Javelin. Use poison javelin on either of his arms until it becomes poisoned. Once his head falls down, use Gallop Lancer to get over there quickly and deal quite a bit of damage. Then, use Thousand Spears. If your attack and weapon are good enough, he should die. If not, wait till the poison kicks in again and his head will fall. Just repeat the process until he dies.

2. Keep the bow on, and walk up to his head. Cast Fine Moment eight times (or until it will not allow you to cast it again). Walk over to his arm, and use Invisible Shot. Once his head falls, press A, and all the arrows should hit him, dealing a massive amount of damage.