After defeating the Pokemon League, you and Hau will help the Ultra Recon Squad capture Blacephalon in Poni Grove on Poni Island. Like all Ultra Beasts, Blacephalon has some pretty incredible stats. It's more of a Special Type Pokemon, with extremely good stats in both its Special Attack and Speed.
You will always find and battle Blacephalon at Level 60. The first thing you can do to try limit Blacephalon is paralyze it with the move Thunder Wave, thus helping your Pokemon outspeed it and also making it easier to capture. The main goal should then be to try to lower Blacephalon's HP as low as you possibly can. However, since it will already do that on its own with the recoil damage from its move Mind Blown, you should try withhold from attacking it altogether and simply try to catch it. 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 make sure to keep track, as the more Pokemon Blacephalon 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, since they have a x5 Catch Rate when used on Ultra Beasts. And don't forget to ALWAYS save before trying to find Blacephalon. It's a safety precaution, one that has worked since the first generation of Pokemon.