Anytime after completing the Ultra Beasts Post Game Storyline, you can find the Legendary Pokemon Necrozma randomly in the Grasses at the end of Ten Carat Hill. Necrozma is very similar to the Ultra Beasts, just with more power and a different ability that lowers the damage taken from Super Effective Attacks. Necrozma will be at Level 75, making it the highest leveled Pokemon in all of the game.
Tips For Catching Necrozma:
Thankfully, Necrozma actually can be paralyzed. This makes it much easier to catch and almost ensures your Pokemon will always move first, since it is so high leveled. After paralyzing Necrozma, try to get it's HP as low as possible. It likes to boosts its Defense and use Moves that have high power with more HP, so make sure to go after its Special Defense and get that HP down in a hurry. Having False Swipe and Hold Back on some of your Pokemon also helps, since it will guarantee Necrozma is left with at least one HP.
From there, after getting its HP as low as you think you can, Ultra Balls are the best way to capturing it. It will usually take a lot of them to catch this Legendary Pokemon, but they are still your best option. And don't forget to save before trying to find Necrozma, just in case you are going for a certain ability or accidentally knock it out.