How to Start Mission 23¶
You’ll receive Mission 23 (along with four other missions) after speaking to Cogita during the postgame mission “The Researcher of Myths”. As the mission name states, a Legendary Pokémon can be found at Moonview Arena within the Coronet Highlands.
1. Go to Moonview Arena¶
Get yourself to Moonview Arena, ideally by choosing to arrive via the Summit Camp when entering Coronet Highlands. Melli from the Diamond Clan and Cabala from the Pearl Clan will waiting for you. Up ahead, Cresselia has taken over the Moonview Arena.
As soon as the challenge begins, Cresselia will apply a confusion effect on you that reverses your controls. So holding will make you move down, instead of up. To counteract this, hold to move up. When you’re near Cresselia, throw something (like a Pokémon) at it to break its shield. Wait for Cresselia to “roar”, then throw a Pokémon at it to fight it.
Cresselia is quite well known for having tanky defenses, but poor offense. That said, super-effective attacks from it will still hurt. Steel-types are recommended to resist Cresselia’s Psychic and Fairy attacks. Normally, Dark-types would be great, but Cresselia’s Moonblast will quickly deplete their HP.
The most annoying thing about this fight is Lunar Blessing, which restores Cresselia’s HP and adds the obscured effect, boosting its evasion. This makes it extremely hard to keep its HP low. Lunar Blessing only(!) has 10 uses, so if you’re patient, you can wait until it runs out.
Otherwise, use moves that can inflict paralysis or drowsiness (eg. Hypnosis, Thunder Wave) to reduce the chance of Cresselia acting. You could also use moves like Dragon Pulse and Dark Pulse, which never miss.
After capturing the flying croissant, as Melli would have it, you’ll receive the Dread Plate, completing the mission.