Xayah is really strong ad carry in late game, she can easy destroy tanks or delete squishy targets, depends on your build.

I'm using 8.5 ad, 15% attack speed, 9 armor and 8 magic resist. You can use your runes, but I really like these with more attack speed. Versus karma, lulu matchups you can use runes with more magic resist.

You have got really good burst, enemies sometimes cant event heal yourself while getting crushed with your feathers. Take that advantage to wait a while before using E.

Dont use your all skill at once, you have to control your feathers. You combo should be Q>AA>W to and then autoattack, to use Xayah full potential