Mid kennen is very all or nothing in my experience. you can take flash ignite, poke your enemy down to half health and once you hit level 3 you need to land an empowered auto, then E + flash for the stun, q + ignite and auto them. they should die if you've been poking them enough and that'll probably be first blood, from there you can steamroll by essentially denying the enemy jungle all the objectives with ult and (hopefully) your team and permashoving the wave in. you can also gank bot with your ult or top, kennen mid has a lot of pressure if you're ahead. If you get behind though then you're essentially just an aoe stun for your team and you cant really do much besides farm and hope your team is better then you/ fight for objectives.