i really think this Champion is extremely valid , and people usually spam his Q and loving staying unseen , where he is really more than that ,
Twitch is like Corki is many ways and Teemo in a way , his passive is like Toxic Shot from Teemo and he is probably the worst to face as an armor user, where it is like Corki,

my runes are basic - focusing n armor penetration and attack speed as he benefit from it 100% dont waste your IP on Attack damage runes as he needs attack speed and armor penetration the most

focusing on offense is the best way to play twitch as he needs every small tiny inch of it to get the job done and fulfill his place,

i start with Debilitating Poison which is his W , its a defensive / offensive spell , most of the time people underestimate Twitch and try to kill him , and its a great tool to crowd control them whether to run or to lock them in , after wards i go for E which Expunge also for being a great poking machine as you will be able to to poke your opponent few times and after wards finish ti off with it !! which a great combo W + E and of course attack speed on the rune page will help you improve your passive and its stacks - Q is on level 4 for me as its very obvious spamming your Q at level is useless as im 100% you cant finish someone off,

Surge - i think it was Made for Twitch - it just suits him so well and perfectly attack speed and the ability power for Expunge is just amazing it works like magic - its the best spell on him i highly recommend it , Flash is amazing escape tool and offensive as well but i think its more useful than ghost on twitch cause , if you are running away Flash and your Q would work faster than ghost

i think Alistar is probably the best ever support next to Twitch , your simply untouchable next to that champion , closing gaps wont work on you and he will push the enemy whether further away or closer to you , locked in - i think its the ultimate combo, try to work you the basic Alistar combo and W to slow and lock them down completely dont rush your W as u really need it if they flash away or try to fight back ,
to Gank with twitch in my opinion never risk it before you have a BF Sword you wont be able to finish the job , level 6 - 7 you will probably have your shoes and Bf Sword combining it with Surge and rune page makes you an early nuke