From The Abyss Cheats (Nintendo DS)

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.

Successfully complete the game and save when prompted. After saving, you will return to
the title screen. Load your cleared saved game, then go into the shop to get a new ring
which adds +15 to every stat. Note: You cannot buy it again if you sell it. The
Bosses in the Abyss are also a lot stronger than they were previously.