After defeating Ultra Necrozma, you will be permitted by the Ultra Recon Squad to continuing using there Legendary Pokemon to travel through Ultra Space. As you do this very thing, there is a chance you will travel to the Ultra Crater and find Celesteela. Like all Ultra Beasts, Celeteela has some pretty incredible stats. It's more of a Defensive Pokemon, with extremely good stats in both Defense and Special Defense.
You will always find and battle Celesteela at Level 60. The first thing you should do to limit Celesteela is paralyze it with the move Thunder Wave, almost guaranteeing your Pokemon will always move first and making it much easier to capture. Fire Type Pokemon fair quite well against Celesteela, as well as Electric Type Pokemon. Try to lower Celesteela's HP as low as you possibly can. A move like False Swipe works great when Celesteela has low HP, ensuring to leave it with least one HP. If you have the Rotom Power that makes it easier to catch a Wild Pokemon, then you should definitely consider using it here as well.
But do keep track, as the more Pokemon Celesteela knocks out on your team, 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 Beast Balls, as they have a x5 Catch Rate when used on Ultra Beasts. 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.