Ultra Megalopolis is a location in Pokémon Ultra Sun and Ultra Moon where you will face Ultra Necrozma.
Proceed to the Megalo Tower’s entrance. Continue all the way up the stairs, and you will meet Ultra Necrozma at the top. This Ultra Beast is extremely powerful and can eliminate most Pokemon in a single blow.
If you want to beat Ultra Necrozma, try using Ghost, Dragon, Dark, Bug, Ice, or Fairy type Pokemon with high affection so they can evade and withstand its attacks.
Pokemon like Mimikyu, which uses its ability Disguise to avoid the first attack, and Zorua/Zoroark with a Pokemon weak to Psychic-type moves as the Pokemon assigned to the last slot of your party will cause Ultra Necrozma to use Photon Geyser, a Psychic move that Zorua/Zoroark is immune to.
Bring Pokemon that can inflict status effects if they are knocked out. Shiitonic, for example, has a chance to poison or put the opponent to sleep with its effect spore.
After defeating Necrozma, it will flee to Alola, and Poipole will want to join your team.
You will then return to the Altar of Moone/Sunne, where a story will unfold. Mina will challenge you to defeat her trial in Seafolk Village.