Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This build you will be able to take over the solo lane 3v3, or 5v5. Teemo's attack speed would be at an amazing 2.1-2.5 depending on your level, runes, and masteries. This is a build meant for ganking.
Once you've unlocked mushrooms at level 6 make sure to place them in the brush, and a lot of them everywhere. They are very useful for predicting incoming ganks and preventing the ganks from happening, as well as slowing your running target(s). And if says a large fight of 3v3-5v5 plan ahead and place mushrooms on the sides of the path. Just in case for a quick getaway.
Teemo is squishy, people love an easy kill so take that to an advantage to bait them in for a gank.
Blinding shot will be a life saver in the end against any hard/speedy hitters. Make sure to use it when they are in your face or as soon as they begin to hit your for the full affect of your skill.
Poison spread this is also useful towards Twitch, but is most affective with Teemo. Shoot as many minions with this as possible, this is more for the frequent clicker.
When buying runes its not just make sure to buy attack speed and Crit%/armor pen.
Something to remember
This build does not make you the strongest character, though it will be scary for other players. Even the ones with loads of health/armor this build should make them cower inside their base. Make sure to always go in with an ally if there are 2 or more players keeping their eye on you.
Also, If you happen to be the one who is being ganked, play it safe find lanes that are open without another player in it. Or if that happens to not be that case, Go ahead and jungle for a min, just make sure to keep an eye out for MIA's and how close they are to the area you are jungling.