Teemo a really good champ specially if you use my build not everyone would like but it works fine for me specially for getting tanks killed or just some other.
And this guild will show you how to get the later game deadly finishers.

Pros:
Good finisher
Excellent harrasment
Can get invisible if he not moves
Can fight really good vs AD dpses duhh blind
You team will be happy cause ya kill tha enemy!

Cons:
Poorly if gets ganked
You will steal kills and your team hates that lolzz
If you got no tank you got a big chance to lose the game
Poor health at start.

All the attack speed runes are handy how faster u are doin faster attacks with teemo´s how better u stack with malady and sword of divine and ur madred will doo alot of damage!
If you always doo like 50 to little damage the Greater Mark of Strength is really handy.
Yeah some extra health can be handy if you are dieing by that last little hit.

The masteries are handy cause that extra damage is handy and you want that 10 procent of faster reviving after death in late game trust me and the buff and xp why?
the exp is becuase you can level easier really important for teemo cause than he can upgrade his skills which he hardly needs and the buff than you keep your buffs longer can save your life trust me on that one.
Soo exhaust and ignite..... than you say why ignite !! i say it can finish off enemies cause if many times only poison not will kill them and if you most fight a mordekaiser with incredible health regen ignite halfs that!!! great not? same with a vlad :D

because malady is a stacker and makes your attacks stronger and if you got 2,292 attack speed you will love malady and the sword of divine and the madred soo much!! and its handy to kill towers and the ice mallet you will need that if you got no tank or if you want to chase someone on the late game. Like blitz he has a speed ability with that freeze he has not a thing of help for that ability hehe and the blood is handy for getting your hp back they will steal from you.

If you got insane attack speed the poison is just the finisher specially on the early game trust me the blind as second is good cause you will easily finish off tryndamere or xin or name it specially ashe to!!!! or caitlyn!!! yay :D
And the running speed i think you know why that as last right?

With the incredible attack speed you will farm like crazy trust me but always stay behind minions cause you got low health and if enemy will try to kill you you cant escape because your stupid minions will block you if your in front of them but if your back you have a chance to escape if they not have a bind or stun lolzz.