Giratina is a Legendary Pokemon that often appeared in cemeteries and lived in the reverse side of the Pokemon World. It was said to be banished for its violent actions, which was shown in the anime, when it attacks Dialga as its revenge for damaging its home while fighting against Palkia. It was shown to transform into its Origins Form whenever it was in its home (Reverse World), or holding a Griseous Orb.
Yveltal was in the Reverse World, on the search for more energy to feed from the Pokemon. However, there was something not right about this place. It didn't seem to have Pokemon in the place. However a electricity attack quickly heading toward Yveltal, but it dodged just in time before the attack hits it. Yveltal then saw that Giratina was behind the attack. Yveltal roared at Giratina to stand down, but the Dragon Pokemon wouldn't back down. Yveltal's claws began to be covered in flames before trying to attack Giratina with it. However, Giratina used it's tentacles-like claws to defend itself, and hit Yveltal on the spot. Then Giratina disappeared by the using the power of darkness.
Yveltal tried to look for Giratina, but it was confused about the place. Then all of a sudden, Giratina hit Yveltal on the spot causing it to fall on the ground. Furious at Giratina for causing a sneak attack on it, Yveltal fired a Dark Pulse attack on his Opponent. The attack hit Giratina successfully, causing it to take a lot of damage.
However, just as Yveltal is about to deliver the final blow to his opponent, Giratina zapped his opponent with a Thunderbolt, shocking Yveltal. Yveltal tries to attack Giratina, but it was paralyzed, preventing it from taking action.
Knowing that Yveltal can't move, Giratina proceed zapped it's opponent with another Thunderbolt attack. Couldn't take another attack anymore, Yveltal has become fainted. Realizing that Yveltal was unable to continue, Giratina roared in victory over his opponent.
While Yveltal has the type advantage over Giratina, it doesn't know the Reverse World well. In addition, Giratina was even more powerful in the Reverse World. The tentacles-like claws is able to protect Giratina from Dragon Claw, and knows in the Reverse World better than Yveltal.