This build will survive suprisingly more than would be expected from Kennen and it will also do more damage than would anybody expect from Tankennen - if he mannage to survive and stack Mejai's enough. He can farm safely and agresively and when he get Zonya he can start killing in teamfights and act as an tank-iniciator. In late game he has not the biggest damage for a Kennen, but he can attack again and again. If he mannage to survive (and he can!) Mejai's stacked power will be huge advantage.

Early
Farm - dont get invovled in champion fights and dont return to base too much. Your Regrow pedant would grant you life restoration you need to farm without returning. When you have enough money to buy both Boots of Speed and Negatron Cloak go home. Second stop would be for Meijai's.

Mid
Get Force of nature for live estoration, great resist and a bit of speed. Now you can withstand lots of magic damage, so go farming where are no meele champions. Hide behind a tank, farm and do some impacts using sequence 1-3-2-1 - for groups add 4. When you have your Zonya you are finished tank. Now you just need to become the DeathTank. You can start harrasing all over the battlefield. Zony will grant you first big improvement in improve in damage and yet it increase your armor against meele.

Late
With Abyssus and Rebadon you wil finally become deadly. If you mannage to survive and do some kills and assists, your Mejai will be full of stacked damage. Your magic penetration and resistance reduction from boots and scepter will multiply effect of your relatively small ability power (for a kennen). You can initiate and join teamfights. You are hard to kill. Dont get involved in one on one fights - if you dont kill enemy with one sequence, he will probably damage you a lot. But you can surprisingly survive most of these situations.

Tip: Good strategy is sometime to pretend fear and run - than suddenly turn around and do second all-out attack.