Basically this build is tho penetrate magic defenses and deal as much damage as you can to multiple opponents while still being able to take a lot of punishment and get away from the enemy.

first I get the sorcerers to give an early magic penetration advantage.

Then I proceed to get zhonya's hourglass to keep up the damage output while making you more durable to coming attacks.

Then a rylai's crystal scepter to keep you durable and giving more damage.

next is a rabadons death cap witch i think is essential to all mages and will give you a huge boost to damage.

An abyssal scepter is another way to bypass the defenses of the enemy very effectively with kennen because you will mostly be close to the opponent when attacking.

last is the Will of the Ancients witch gives a big bonus to Ability Power and will even give you and your allies spell vamp and extra ability power.

my runes are very simple mage runes and they go as follows:

Greater Quintessences of Potency (Purple)

Greater Mark Of Insight (Red)

Greater Seal of Force (Yellow)

Greater Glyph of Force (Blue)

i follow a simple 9/21/0 mastery page hitting all the way to Archaic Knowledge in the Offense tree and then switch to the Defense tree hitting a lot of the major Defense Masteries.

WARNING: Farm when you can until you hit level 18 go for last hits on minions when possible.