Kayle is an EXTREMLEY versatile Champion. She can play in literally any position. I prefer to play her middle lane and top lane, with a preference towards middle because of brush location. Jungle is the next most viable position for her as her E allows for nice clear times and the W for a relatively safe jungle. Support is next in the lanes I would play her, the heal and ultimate is very helpful in team fights. ADC is not recommended because her range only lasts 10 seconds but it is still able to be done and works well if you know your support well. Now for the Meat and Potatoes.
Yes it is ALL Scaling runes. Think about it. Scaling might only catch up too Flat at level 12 but you are only level 1-12 for maybe 10-15 min. that is 1/3 of the game. The rest of the game you will be behind someone who is using scaling. If you argue that Flat is better because you win lane, then I will counter with Two points. First and Foremost, If you play passive and farm no amount of AP will help them kill you. You have an E that is ranged sit at tower and use it. Secondly It is your game I am only providing suggestions that I have found useful. You don't have to take them. My Point with the runes is that Late Game is more important than Early Game. You can lose early but if you win Late you win the game.
Here as you may well know the far right column in for AP champs, and the Second left column is for AD. I do not take the increase AD by 5% because this build does not build a lot of AD(BORK). I Do take the AP increase because of runes and my items are slightly AP(Nashors). The Far left side is good for Kayle because it strikes the right balance between attack then use spell and use spell then attack. Kayle does both at the same time and individually depending on the order that you use skills in a fight. The right side is for sustain in lane and out with the HP increase from minions slain and for champs killed. Arcane blade is taken because it is great for AP champs and with Attack Speed it greatly improves the damage.
Dorans Ring start to give Flat HP. Mana regeneration and mana gained on kill. The HP and Mana Pot is for even more lane sustain. Berserkers greaves gives attack speed which is the core of the build. The Furor Enchantments are for chase capability, and also the ability to run away. If for instance you use Q to slow the enemy either running away or running towards someone they get slowed and you get a movement speed increase from the Furor boots. Ruunans Hurricane not only applies OnHit effects to all targets hit which include but are not limited to, Kayles Passive, Wits end Passive, Nashors passive, Bork Passive and Lifesteal, but it also increases attack speed. Wits end gives Magic Resistance for all occasions and attack speed, Plus it gives the Magic Resist steal which combos very nicely with Kayle Passive. BORK Provides some minor attack damage to help out Ruunans and E power. E scales off BOTH AP and AD. BORK also provides attack speed, Lifesteal, and the most important thing that also goes nicely with Kayles Passive. Now you have armor shred Magic Resist shred AND HP shred. Nashors Tooth Gives Attack Speed, Ability Power, and the Passive Magic Damage. GA or BT are the last items. If you want more damage AD wise with lifesteal bonus BT is the way to go. If you want more tankiness and a 3rd life(Kayle has Ultimate) then GA is your thing. Do Not Sell Dorans Ring Until You are working on the Last Item GA/BT.
You max E first for the increase in splash damage and single target damage. It is The Main skill used in this build because the entire build revolves around Attack Speed and Shredding the HP Magic Resist and Armor of the enemy so that you and your team do maximum damage to them. Q is maxed next for the slow and damage. Then W for heals for late late game.
If you don't know what the champion does that is sad.
Q Slows enemy and deals damage.
W Heals and Ally or Yourself, And gives increased movement speed.
E Makes Kayle Ranged and gives her extra damage to target hit and surrounding Enemies for 10 seconds.
R Makes Target invulnerable for 2-3 seconds.
In General This build focusses on Attack speed and putting out the most DPS possible while E is active to the most targets possible while also making your allies deal more damage to the enemy as well. NOTE: You do a lot of damage do not be afraid to go in and start swinging.
I Main Kayle Middle in ranked all the time. She is Practical the only person I play in ranked. She is extremely viable and no one ever banns or picks her so do not worry about that. She is also not very easily countered. There is not a single middle lanner conventional or not that I do not feel comfortable fighting. The only problem that my Kayle has and that any team will eventually has is Nasus. His wither is very strong. You might say but what about Exhaust. Well, it is not where near as big a threat as wither is because with Exhaust you are still at roughly 1.6-1.8 attack speed which is still respectable.
Pros / Cons
Pros. Kayle does a lot of damage she is very good in team fights and her ultimate is a game changer. Her farm is great with E. Her escape and chase potential are great.
Cons. She is kind of squishy. Early Game.
If people are low heal them, if someone does more damage than you late game(not likely) Ultimate them. Attack anyone that comes near you tank or not with your E turned on. It will splash and Ruunans will do its work to wreak havoc on the enemy team. If the enemy ADC or Mid Lanner is strong focus them flash in Ultimate yourself and do work. If you come here and read this guide for teamwork then you do not deserve to play this game on a competitive level and you need to go practice. This is for specific team fight abilities...but really skill sequence---E-Q-W-R-E Win team fights...Not hard.
Sit behind minions and farm using E. Use E to hit minions when they are low.. Do not pop E until one minion is low or you will lose the timer on it and be melee....(not fun)....If you are winning lane hit the caster minions first. One hit on each caster and they are dead. it also zones out the enemy lanner even more because of the splash from your E. There You have now mastered Kayle. Go out and have FUN.