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 fourth Ultra Beast you are set to capture is Celesteela. Like all Ultra Beasts, Celeteela has some pretty incredible Stats. It's more of a Defensive Pokemon, with extremely good stats in both its Defense and Special Defense Stats. Luckily it's only a Flying/Steel Type, allowing it to be paralyzed.
Here Are Some Tips For Catching Celesteela:
Like many of the other Ultra Beasts, you are set to catch more than one Celesteela, two to be exact. It can be found randomly in the Haina Desert. Since it's at Level 65, a player can easily put a Level 60 Pokemon at the lead of their party and use a Repel to quickly find Celesteela in this area. 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.
Right as the Battle begins, Celesteela's already great Defense will increase by two stages, making it a serious tank. The first thing you can do to limit it is by paralyzing it on the first turn in battle, making it easier to capture later in the battle. And since it has Autotomize as one of its Moves to boost its below par Speed, this will help here too. Steel Type Pokemon actually fair well against Celesteela, since it mainly uses Grass and Normal Type Attacks. From there, try to lower Celesteela'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. The lower it's HP is, the better chance you have at catching it.
But keep track of the more Pokemon Celesteela 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. But don't be afraid to use Ultra Balls on Celesteela, if you'd like to save your Beast Balls for your next challenge.
And don't forget to ALWAYS save before trying to find Celesteela. It's a safety precaution, one that has worked since the first generation of Pokemon.