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 Forest and find Kartana. Like all Ultra Beasts, Kartana has some pretty incredible stats. It's more of a Physical Sweeper, with extremely high stats in both Attack and Speed.
You will always find and battle Kartana at Level 60. The first thing you should do to limit it is paralyze it with the move Thunder Wave, almost guaranteeing your Pokemon will always move first and making Kartana much easier to capture. Fire Type Pokemon fair well against Kartana since it mainly uses Grass Type Attacks, but make sure not to hit it too hard as it's pretty frail defensively. Try to lower Kartana's HP as low as you possibly can. A move like False Swipe works great when Kartana 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 Kartana 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 Kartana. It's a safety precaution, one that has worked since the first generation of Pokemon.