I main Kennen, and used to build him pure ap, however in team fights he would die very quickly. One day I decided why not get some hp, and how does one go about doing that? WARMOGS. Kennen plays very well as an Ad/Ap hybrid.

Using Kennen takes some time and getting use too, he can be VERY annoying to play against if you know how to use him right. Remember, letting the opponent overextend is your best weapon.

Pros:
good damage early game
great team fighter late game
good AD and AP
3k+ health
can take damage

Cons:
He's squishy until you get warmogs
you lose quite a bit of ability power by getting Warmogs
Your ultimate get's a harsh damage drop by not building full ap

I start my build by getting Dorans right, good health and ap altogether, you sell it later to help buy whatever item is needed (I almost always end up trading it in to finish rabadons death cap). Then i get the boots like usuall into a rabadons. At this point, unless you've been heavily fed, your probably lvl 11. At this lvl team fights begin to occur, so I save up for warmogs than depending on who's on the other team you'll have to make the choise weather to get a void staff or a crystal sceptor, void staff if the team is tanky and you need some penetration, and sceptor if you need more damage and health and people tend to flee at the sight of you. Finally I end the build with a lich bane and the hourglass. Lich bane is your best friend late game. Having a good 480 AP Kennen's normal attack will do 500-700 damage after using an ability, which is very useful. The Hourglass is nice, and I like getting it more than a second death cap mainly because it allows you become invulnerable for 3 seconds. In team fights at the end you can zoom into a croud and activate your ulti, which normally is the worst thing you can do. However with the hourglass you can make yourself involnerable while the enimy team tries to figure out why your not dying when all of them are punishing you.

Sometimes your doing extreamly well in the beggining and the enimy champion just can't seem to do damage or fails at there skillshots so warmogs arn't needed as much, if so you can get you sceptor/void staff first before getting warmogs.

Kennen is a very good creep killer once you get rabadons death. using your e skill to zip past creeps and then w to finish them off, it's a good way to get money to buy your items if your having a difficult time killing champs, and it's a great way to lvl. Once you get your death cap you will almost always 1 shot creeps, if you can't just buy an amplifying tome, it build into your Rylan's sceptor anyways.

Kennen is very useful, he can get kills, and ***itst depending on where you are in your build. Kennen at lvl 3 will 3 hit stun ppl. Generally I wait for them to get a little distance from there turrets and then ingage with my e skill, tuning into a lighning bolt and zooming past them and the minions there hiding behind. Next I would immediately activate my w skill, at this point there at 2 marks, and normal champions would begin to run at this point, this is where you q and then you and/or your partner runs in and finishes him.

He is very good at saving other champions. By running in and using the same combo (if your q lands) you can stun your opponent in less than two seconds stunning them in place as your alley escaps, you can then either continue fighting or begin fleeing yourself.

In team fights Kennen is very useful. If bunched together it is sometimes optional to e into the group and then use your R which is like fiddlesticks ultimate hitting every opponent for a good amount of damage and then QUICKLY activating your hourglass so you don't take damage. This is usefull because if they don't scatter, or can't scatter fast enough you will stun them, your ultimate putting a mark on them every second and they already have 1 mark. This gives you time to escape or continue your pursuit and gives your alleys a way into the fight.

Early to mid game if kennen is dominating his lane (I usually try for mid) it is very useful to watch your map for overextention (when your enimies are pushing or right next to a turret. This is because Kennen is probably the best harraser in the game, walking up to the lane and hiding in a bush. It's always important to ping out a certain champion and kill them, don't get greedy and try to take both, because then you may end up getting none. Walk in until your in range of your e, then e in, w, and then q the squishyer one and your group can destroy them. It's not always important to get the kill, Kennen works well as an assister.

I like to get exhaust and ghost so incase I stun someone and get them low on health they won't escape me. However, if Karthus is on the other team, not matter what champion I play, I switch out ghost for heal, because his ultimate will hit you anywhere on the field, and I don't use it unless he is using his ulty.

Another very useful summoner spell is ignite. I love ignite because early game it is very easy to get an opponent to one bar of health, but very difficult to finish them unless they are literally at your turret, this allows you the kill, and the chance to farm on some nummy minions early game. However in late game it's less useful than ghost imo because normally at end game if someone is low health they die or escape in such a way via there teamates that ignite just isn't feasable, because even with warmogs, rushing through the entire team to get to one champion to ignite them just isn't going to happen unless you die, then it just becomes another four on four without you there to harras.

There are two types of Kennes, the harraser, and the pusher. A harraser allows the enimy near there turret allowing them to overextent and then activates his three skill combo locking them in place while doing major damage. This is very useful, it's how I play when I'm up against a kaitlan or a Lux. The pusher forces the enimy to hide behind there turret by continuously activating his combo and severly damaging the enimy anytime they leave turret range allowing you to lvl past them and put some damage on there turret.

If you are the pusher you always need to keep a keen eye on your map to see if anyone is missing. If you ever look over and notice someone is gone or mia, it's not critical to rush back to your turret immediately, you just need to remember to not use your e until they are found. This is because Kennens e can outrun almost anything. Now it's important to note not to panic use your e skill because you may be stunned or feared. I always walk back towards my turret if I get ganked, and then after the attack me or use there first skill I e ush past them, w, and then q backwards to stun them in place.