Use the following strategy to make the final Boss battle (Abyss 8 - Floor 4) easier. Make sure you're at least level is 55 or more and that you have a very good spear and bow. The first part of the Boss will be a giant cocoon that's not very difficult, just very annoying. Equip the bow and, with t he skill Fine Moment set, walk up to the cocoon and use Fine Moment eight times (or until you can't cast it again). Then, Invisible Shot until it is about three-quarters dead. Finally, press A and it should die. An intermission sequence begins of the world being absorbed into darkness. Once the darkness clears, you will see the final Boss (The Abyss King). There are two ways to defeat him:
For the first method, equip the spear and have the skills Gallop Lancer, Thousand Spear, and Poison Javelin. Repeatedly use Poison Javelin on either of his arms until it becomes poisoned. Once his head falls down, use Gallop Lancer to rush and deal some serious damage. Finally, use Thousand Spears. If your attack and weapon are strong enough, he should die. If not, wait until the poison kicks in again and repeat the process until he dies.
For the second method, keep the bow on and walk up to his head. Cast Fine Moment eight times (or until you no longer can't), then walk over to his arm and use Invisible Shot. Once his head falls, press A and all the arrows should hit him, dealing a great amount of damage.