4 KAYN TIPS FOR ANYONE LOOKING INTO THIS CAMPION
(if you want, try everything I'm going to say in practice tool.)
#1: do not worry about how much of one champion type you have hit. (getting that exp for passive)
kayn doesn't actually record how much of range or melee you have hit and gotten the orbs from all that matters is the last type hit. so go into a practice tool game add a soraka bot and get 99% full on training dummy's (melee) then hit soraka for the last 1% (ranged) the way this is worded might make almost everyone think that your going to get rhaast but in reality you will get SA every time cuz it only recognizes the last hit.
(TLDR: make sure the last champion type hit fits with the form you want.)
#2: dusk blade is your go to item on both forms
your ult resets your dusk blade, giving you a lot of dmg. also it strong for the vison game. 10/10 item on kayn.
link is to a build path of how I have played about 40 kayn game and it work's very well: https://cdn.discordapp.com/attachments/157073119587205120/349264753526439947/Untitled.png
#3: take full lethality in your runes.
kayn's early jungle dmg with the bonus 55 on his Q opens up his options a lot, he doesn't have to take AD to keep his jungle speed up since its built right into his kit. this gives you a huge spike of dmg as the game carries on, since you start the game with 30 lethality.
#4: I prefer storm raiders surge but you might be a thunder lords guy/ girl.
both are really good on him it really comes down to personal preference id suggest you try both, thunder lords gives you more burst to your burst and is extremely nice on SA since the passive will work on thunder lords. but I like the speed and sticking power of storm raiders.
for the waveclear and the amped proc with duskblade, and it also deals as a sort of utility tool to split as S.A since I heard it on the reddit threads that it is a quite interesting off-meta build.