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 Jungle and find Buzzwole. Like all Ultra Beasts, Buzzwole has some pretty incredible stats. It's more of a Physical Pokemon, with high stats in both Attack and Defense, but it's also bulky with nice HP.
You will always find and battle Buzzwole at Level 60. Like most Legendary Pokemon, it's best to try to Paralyze Buzzwole with the move Thunder Wave in order to make it easier to capture. From there, try to lower Buzzwole's HP as low as you possibly can. A move like False Swipe works great when it has low HP, ensuring to leave with at 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 Buzzwole 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 Buzzwole. It's a safety precaution, one that has worked since the first generation of Pokemon.