After defeating the Elite Four and finishing the main storyline, you will be able to start battling and catching the Ultra Beasts through the Looker Storyline. The first Ultra Beast you are set to capture is Nihilego, the same Ultra Beast you had faced in a number of battles in the main storyline. Like all Ultra Beasts, Nihilego has some pretty incredible Stats. It's more of a Special Pokemon, with high stats in both Special Attack and Special Defense, but it's also quick and bulky with nice Speed and HP. Luckily it only is part Rock Type, which means a player can disable it with paralysis.
Here Are Some Tips For Catching Nihilego:
Unlike many of the other Ultra Beasts, you are only set to catch one Nihilego. It can be found randomly in the Wild in either Diglett Tunnel or Wela Volcano Park. Since it is at Level 55, a player can easily put a Level 50 Pokemon at the lead of their party and use a Repel to quickly find Nihilego in these areas. A player will know they are near this Ultra Beast by a change in the music and a not-so-far off cry from it.
Like most Legendary Pokemon, it's best to try to Paralyze Nihilego first thing so you can always go first. Right as the Battle begins, Nihilego's Special Defense will increase by two stages, so make sure to go after its poor Defense with Physical Attacks. From there, try to lower Nihilego's HP as low as you possibly can. Moves like False Swipe and Hold Back work great when it has low HP, ensuring to leave at least one HP.
But watch out the more Pokemon Nihilego knocks out on your team, as its Highest Stat will increase every Time it does so because of its Beast Boost Ability. Because of this, like all the other Ultra Beasts, it's best to try to capture it as quickly as possible with the Beast Ball, since it has a x5 Catch Rate with Ultra Beasts.
And don't forget to ALWAYS save before trying to find Nihilego. It's a safety precaution, one that has worked since the first generation of Pokemon.
Or you can use Zygarde and have him use Bind on it, use a bunch of Hyper Potions and what not on it and it'll slowly whittle down the HP of the Ultra Beast.