Because this is a teambuilding contest centered around Kartana, I decided to pick the SD Z-Giga Impact set since it has been growing more popular and it hits everything pretty nasty. Pretty standard.
When I think of things that threaten Kartana the most, I think of Bulky Grass-Types and Volcarona. This AV Torn is a bit unorthodox, but it lives a +1 Flamethrower from Bulky Volcarona with a 0% chance to be 3HKOd and 2HKOs Volc in return (OHKO with rocks). Focus Blast allows Torn to continue to hit Steel Types and Rock Types
Next, I wanted a rocker - one that could threaten Magearna. While Heatran still fears Focus Blast from Gear, Gear also fears Magma Storm in return. I chose Calm + Leftovers for longevity. Taunt also helps stallbreak things like Pex which this team struggles to break unless teammates can set up.
Specs Keldeo was up next because this team would have gotten decimated by Ash-Greninja. Keldeo is easily one of my favorite Special Wallbreakers in the OU tier and one of my favorite Gren counters. Icy Wind punishes switch-ins from things like Mega-Venu, Mega-Zam, and the Latis.
Now the team needed an answer to Koko and an Electric answer. Lando-T was my go to because... its Lando-T. Stone Edge for Flying-Types which threaten the team as well.
Mega Mawile rounds the team out as I still needed a Mega Pokemon and I like to balance my Physical and Special offense on teams I build. Knock-Off and Sucker Punch are for hitting dangerous Psychic-Types like Zam and Lele.