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 Ruin and find Guzzlord. Like all Ultra Beasts, Guzzlord has some pretty incredible stats. It's more of a Physical Tank Pokemon, with extremely good stats in both its HP and Attack. Literally, Guzzlord is like Chansey or Blissey, but one that can actually attack.
You will always find and battle Guzzlord at Level 60. The first thing you can do to try limit Guzzlord is paralyze it with the move Thunder Wave, making it easier to capture later in the battle. Steel Type Pokemon actually fair quite well against Guzzlord, since it mainly uses Steel and Normal Type Attacks of its own (just watch out for Hammer Arm). From there, try to lower Guzzlord's HP as low as you possibly can. A move like False Swipe works great when it has low HP, ensuring to leave it with at least one HP. The lower its HP, the better chance you have at catching it. But don't get Guzzlord's HP too low, because it loves to use the move Thrash, meaning it could actually make itself faint from its recoil damage with confusion. 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 Guzzlord 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 Guzzlord. It's a safety precaution, one that has worked since the first generation of Pokemon.