The thing for this build is: MUSHROOMS EVERYWHERE! put mushrooms in all bushes so when enemies escape and run into the bushes under half health they die from your massive magic damage and poison from your mushrooms. use teleport to get to all places on the map by teleporting to mushrooms.If you think you can uphold the stacks on mejai's then instead of malady get mejai's.
Always hit the enemy with toxic shot before hitting them with blinding dart it will cause more damage over time and after toxic shot ,blind them with blinding dart.I don't think that teemo needs ghost or flash do to his move quick and ignite would be much better for more damage over time.If you are having problems from defending your self from powerfull AD enemies buy thornmail instead of guinsoo's.If you are having problems from defending your self from powerfull AP enemies buy force of nature instead of guinsoo's.Always put more than one mushroom in each bush so if somebody steps on one you can teleport on the other one and finish them, and also for extra damage. if your placing mushrooms on lanes then be sure to put them off the minion tracks.
Hope you enjoy playing with this build and have fun :)