Gore-igami
Kartana@Life Orb
Jolly nature
Beast Boost
252 Attack/252 Speed/6 HP
~Leaf Blade
~Smart Strike
~Sacred Sword
~X-Scissor/Night Slash
Kartana is pretty cool. Would likely be my favorite Ultra Beast were it not for Nihilego. So yeah, you got Leaf Blade and Smart Strike as your STAB moves of choice. Sacred Sword is very nice coverage, and the last slot, you could go either way, be it Night Slash or X-Scissor. Pick the one that suits your needs best.
Item powers up Kartana's moves. Ability is the only option, but as it's essentially Moxie, there's really no room to complain, is there?
Other options:
*Obviously, Kartana's got Swords Dance- I'd be very surprised if it didn't- and if you can find room for it in its moveset, then by all means. There's no kill like overkill.
*Kartana gets Calm Mind, which does in theory help its lacking Special Attack and Special Defense, but... its special movepool is rather lacking. And when I say it's rather lacking, I mean Vacuum Wave, Air Cutter, Air Slash, Hidden Power, and Round. That's literally it. No Energy Ball, no Flash Cannon, no Giga Drain, none of that good stuff. I would say pass on Calm Mind, honestly- Kartana's got way better options.
Abilities:
*Beast Boost: When Kartana KOs an enemy, Kartana's highest stat goes up one stage. Only takes into account what the stat is at the start of the battle, so buffing another stat higher than the highest stat won't make it get buffed by Beast Boost instead. In Kartana's case, this will always function as Moxie, only buffing its Attack, barring something ridiculous like an Attack-reducing nature and no Attack EVs. Its Attack is that high.
Partners:
If it's got good Special Defense and/or doesn't mind switching into a Fire attack, odds are it's a good partner for Kartana. Tailwind/Sticky Web support could also work- 109 Speed is great, but there is some stuff that outspeeds it.
Counters:
Chandelure and Alolan Marowak can switch in with impunity, unless Kartana's running Night Slash, and their Fire moves are gonna rip through Kartana like the tissue paper it is. Fire-types in general, actually, generally do pretty well against Kartana.
Opinion:
I like Kartana. Cool design, nice stats, really gets the job done in battle.
Prediction for next week:
Wobbuffet.