Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
The reason I have choosen a AP build for this character is because the AD/AS build seems to just make kennen a little brat who has a lower output of damage then he should.
now to jump into the Srs bsns-
I've choosen a build for him which is 21/0/9
You've probally noticed its not a standard caster build and that i went all the way down to the bonus damage this is just for well... bonus damage not much explanation needed.
The skill sequence I've choose for kennen is mainly start with w/q/w/e/q/r from there out... I could really care less how you do it.
The reason you get your Electrical surge (defualt w) first is simple -> on your 5th attack you will not only be doing bonus damage, you will be putting a mark of storm on the enemy which will than allow you to use your active of the ability adding your second mark of storm, i've found that this combo start game will take out 2-3 bars of your enemies health.
The Thundering shuriken (default Q)as your second ability just adds a harass and a possible faster stun if needed, I would mainly harass and use as a aggresive stun mixed with your Electrical surgeif your lanning agianst a fiddlesticks who likes to drain alot, basicly hes just wasteing mana now.
Using Lightning rush (default E) properly is easy to some harder for others usually if your doing well you will get extermely cocky and try to charge in there for a quick stun and kill the guy. But i find the purpose of this move is definitly geared to putting out a quick combo with Electrical surge. Which is used well in either a team fight to put out some quick damage or killing a group of creeps swiftly
Finally your Slicing maelstrom this is so easy to get a 1v1 done or a kill start game just using the same combo E/W/R and if they try to run Q the problem with it in a team fight is this may not hit a target that you want it too , an advantage is it will hit stealth units and add a mark of storm in which you can electrical surge quickly and possibly get a kill. I've done this multiple times to shaco's and twitch's just passing through a missclicked ult or one i've just used to kill an enemy 1v1 or so forth and than you can expect a very rude message.
Next on our list is my choice of runes.. yes magic pen mainly because the more you have the more damage your putting out with your spells. point&case
Now to the item build....
You should start off with dorans blade & a health potion even though its not a damage build i feel that the blade is definitly better start game.
The first time you go back you should definitly have enough to get a Zhonya's ring starting with this item puts you at a huge advantage AP wise but if for some reason you have to back try to make it back with 1600, enough for your needlessly large rod.
By the time i go back the second time I'm able to grab a Rylais and a pack of Sorcer's treads more AP and some penetration with movement speed to top it all off. (if you cant get a rylais and feel movement speed is a necesity than sure grab boots.)
I cant stress enough to keep farming and helping in team battles since your instinct will be to back alot but keep going for your last 2 items (besides the zhonya's but ill get to that)
Finally if youve made it this far in the game you can get another zhonya's just sell your doran's blade tell her its not you its me and grab a much more attractive Zhonya's or .. if you feel like your really daring and just want to replace your dorans grab a haunting guise... more magic pen..and about 1/3 the cost