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 Plant and find Xuriktree. Like all Ultra Beasts, Xurkitree has some pretty incredible stats. It's more of a Special Pokemon, with an extremely high stat in Special Attack.
You will always find and battle Xurkitree at Level 60. Grass Type Pokemon are best against Hurkitree, as it won't be doing much damage to them. During the battle, try to lower Xurkitree's HP as low as you possibly can. A move like False Swipe works great when it has low HP, ensuring to leave Xurkitree with at least one HP. Since you can't paralyze Xurkitree, it's best not to status condition it at all, unless you can induce sleep when it has very low 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 Xurkitree 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 Xurkitree. It's a safety precaution, one that has worked since the first generation of Pokemon.