Defensive Steel Types like Celesteela can come in on most attacks Kartana uses and threatens Kartana back with Flamethrower. Skarmory can also do the same thing with the added boost of Roost, but can't threaten Kartana efficiently except with niche moves like Counter.
Other scarfers that can KO with special attacks (ex. HP Fire Scarf Greninja, HP Fire Scarf Serperior)
Bulky Grass types like Amoonguss and Tangrowth can wall Kartana's Leaf Blades.
Bulky Volcarona take both of Kartana's STAB attacks and threatens it with a STAB Fire move, as well as being able to come in to Kartana multiple times if it has Roost and Stealth Rock is not up on the field.
Other Scarf Kartana can speed tie with Scarf Kartana, if you feel risky enough to win a speed tie and get a KO with a Sacred Sword. Although you need an entry hazard like Spikes up (or just prior damage on the opposing Kartana) because 1 Sacred Sword isn't enough to KO Kartana from full HP.
Hope this helped!