Q&A for How to Get Mewtwo in Pokémon Ultra Sun and Ultra Moon

Use an escape rope to teleport to the entrance of the cave, or use a Max Repel and find your way out without many (or any) encounters.

You're going to need to teach an Absol both False Swipe and Thunder Wave. TM54 (False Swipe) is obtained after beating the Melemele island Kahuna, Hala. TM73 (Thunder Wave) is in the top-left corner of Malie Garden, but you're going to need to fight some trainers while crossing the bridges there. After that, you just need to get the Master Ball (Gladion gives you one while fighting the Aether Foundation), and you're set.

Usually, hacked Pokemon will already be level 100 and have the purple Pokerus icon by their name.

If you have a Master Ball, use that. Otherwise, try using a Quick Ball on the first turn. If you don't want to do that, weaken Mewtwo and paralyze/put it to sleep, and use Ultra Balls. If you know that at least 30 turns have passed, try using Timer Balls.

Yes, in US/UM there are tons of Pokémon you can find. However, most of the legendary Pokémon are only able to be found after you get the primary legendary of the game. You can often find all kinds of rare and legendary Pokémon by using the ultra wormhole.

No. You can only visit the Ultra Deep Sea to progress the story. Once completed, you cannot return. There are no Pokémon that can be caught during the plot necessary trip.

Legendary Pokémon found within Ultra Space will respawn until caught. If you have knocked it out, encounter it again. However, if you caught it, you will not be able to encounter it again.