The Summoning

After the main story concludes, you will encounter and battle the legendary Pokémon Rayquaza. Use general catching techniques: lower its HP to 1 with False Swipe, inflict status conditions, and then use Ultra Balls (or Dusk Balls at night). Rayquaza will be Level 70.

After capturing Rayquaza, you must place it in your party. If your party is full, you will choose which Pokémon to replace.

Zinnia will then teach Rayquaza Dragon Ascent, which is required for its Mega Evolution. Unlike other Mega Pokémon, Rayquaza does not need a Mega Stone to evolve; only Dragon Ascent in its movepool is necessary.

You will then battle Lorekeeper Zinnia a second time. Her team consists of Goodra Lv. 60, Tyrantrum Lv. 60, Altaria Lv. 60, Noivern Lv. 60, and Mega Salamence Lv. 62.

Mega-Evolve Rayquaza and use its powerful moves, such as Dragon Pulse, to defeat Zinnia quickly. You can also try using Dragon Ascent.

Following the battle with Zinnia, you will pilot Rayquaza into space to confront an asteroid.

From the remains of the meteorite, the legendary Pokémon Deoxys (Normal Form, Level 80, Psychic-type with Pressure ability) will appear and initiate a battle.

It is highly recommended to use your Master Ball to catch Deoxys, as it is one of the strongest Pokémon in the game and this is a rare opportunity to obtain it, especially since you cannot save before this encounter.

Upon returning to earth, Aster will give you a letter from Zinnia, concluding the Delta Episode.