I do not claim myself a champion of the Crystal Scar and Kennen, but this is my first guide and focuses on the dominion part of the game. Follow this at your own risk :)

The Runes I've chosen for this guide is plain simple stacking as much ability power and spell penetration for damage. I do not focus on the defesive part here.

I've chosen a 9/21/0 mastery build for defensive purposes. Kennen is a squishy, area of effect (AoE) champion who's primetime is in the team battles. Thus Kennen is often the target of the enemy team and requires some defensive attributes. I've picked the damage reducing masteries in the defensive tree and the ability power gains in the offense tree.

[*] Mercury's Treads - I get these for the movement speed to get the first turret fast and the magic resist for defense.

[*] Thornmail - This is a great item to get for Kennen as it provides a lot of armor and gives the passive 30% damage return from basic attacks.

[*] Haunting Guise - A great offense and defense item. Provides ability power and spell penetration and additional 200 HP.

[*] Rabadon's Deathcap - This is a really important item. Provides a huge amount of ability power (+140) and a 30% overall increase of ability power. Pure damage item!

[*] Rylai's Crystal Scepter - Both a defensive and offensive item. Provides a great HP boost of 500 and 80 ability power. The passive, movement slowing, helps both Kennen and his team mates as a utility.

[*] Zhonya's Hourglass - Ability power for offense, +50 armor for denfense. The passive here is a real lifesaver. It will put the user in a stasis for 2 sec, making the user untargetable and invulnerable to damage to save you from those nasty nukes.

At the start of the match I get 1 point in Thundering Shuriken (Q), Electrical Surge (W) and Lightning Rush (E).

I max out Thundering Shuriken first for the single damage skill shot. Then I max Electrical Surge, wich is Kennen's "main ability". This ability will deal damage and stun opponents with 3 Mark of the Storm. Last I max out Lightning Rush. At level 6, 11 and 16 I get the ultimate skill Slicing Maelstorm, a really effective AoE ultimate.

For the Summoner Spells I choose Ignite for that extra single target damage and Flash to be able to move more quickly and efficient on the battlefield.

Kennen is a well suited AoE champion for the team battles. With massive AoE and quick manuverability Kennen is a champion to watch out for.