The element of Dark/Darkness is strong against Light. If an opponent is aligned to Light then this element will give them pain big time.
Whirlwind Hit - Dark-based finisher. Found in the Mystical Garden. It is a strong finishing attack, but Gibari's other dark attack is obviously better. Still, it can be useful at times.
Nemesis Fall - Dark-based finisher. Found after defeating Brokolis. It is better than Gibari's previous dark finisher, but not too useful through the rest of the game.
Sforzando - Dark-based finisher. Found in the Ancient Library of Magic. It is another powerful finisher and a pretty cool one to boot. Definitely useful throughout the rest of the game.
Diminuendo - Dark-based finisher. Found in the Outer Dimension, when fighting the eight wave of enemies. Although it is not as cool-looking as Sforzando, it is a lot more powerful. Keep both in your deck at all times.
Crescendo - Dark-based finisher. Found after defeating Galatea. It's a good finishing attack overall. Find some room in your deck and leave it there, or just replace it with weaker finishers.
Shadow Wings - Dark-based finisher. Found in the Lord's Mansion (make sure to pick it up on your first trip there). Pretty useful finisher that should be used for a while.
Dark Embrace - Dark-based finisher. Obtained from the boss fight in Nekton.
Shadow Gate - Dark based finisher. Obtained upon defeat of Geldoblame.