Well to clarity i just answer that before to other user but its optional i use cause i want to harass my lane and on team fights it had helped me alot.

For the AS ill buy malady first since i start with Vampiric secpter to hang on on lane and the Last whisper for those 40% ArP(armor pen) and more speed.

If you do this build; boots of mobility, malady, last whisper, Infinity edge, phantom dancer, Black cleaver or even bloodthirster would have a nice as and ad.

About defensive items, since twitch is a squishy champ cause its weak on health i don't recomend to waste money on defensive, belive me i had tryed alot of combos items and this one is the best so far i have movement i have speed i have ad i can kill a tank with this build.

On the other hand i wanna try the crit build its the only one i never did could you leave here the link plz.
thanks for the coment ;)

OK i am not an expert on this champ but ehre it goes.

1st of all, why do you need Clarity for? I played Twitch and had no mana problems EVER.
I prefer the crit build for him, since your AS in early game is low.
As you go Malady->PD i would suggest leaving that Whisper for later game.
Also his AS cap is 75% if you count 100% from ambush and 80% from ultimate. Dont know how vital that is but if you go for Malady+PD+Whisper you will go over AS cap during your ultimate wasting some potential. On the other hand you will farm faster...

No defensive item on twitch? Hmmm you will be extremaly squishy.

Yes but i can't other item who provide that 50% AS and with that lifesteal witch is nice and the bonus of dmg witch is nice too in my point of view but everyone does what want i prefer speed boost on begining, thanks for coment ;)

this will be MUCH better after the malady remake so +1, but with the current malady you would get a minus.. i saw a lvl 15 or so use this exact build in a smurf and i worked well though... but mainly cuz he was on my team and my ap shaco got him fed ;)

As you could read i said i try many builds and i know all about that!

And yes the Mastery you said its a bit better i'll change to that one ;)

I made this guide cause ppl are in doubt of going for crit and AD or AS and AD, well i can tell you AS and AD its ALOT better and are the main stats for twitch.

Responding to the Clarity thing imagine you after killing in a team fight using this combo; Ambush+auto-attack+SnP+Poison(slow).. still 2 up and running... in chase(if u got those precious boots movement 5 out of combat u can do this else u can't)so your in pursuit Poison(slow)+auto-attack+expunge.
Now you are almost out of mana but you can harass tower or inib but if they are getting out by now you must go to base get mana else you gonna die cause another fight will acour and you don't have mana :c
With clarity you can stay there and give some mana to your team mates and continue to kill and harass, im saying this cause once i didn't use clarity and had to go alot of times to base, since i use it lol they just keep coming and i keep ill them as i'm gonna show you my score table.


http://img256.imageshack.us/i/semttuloiz.jpg/

Your summoner spells are rather passive. Flash, Ghost, and Ignite are generally superior options to Clarity and Exhaust. By level three, you already have a slow, so Exhaust will be dead weight at that point.

Your reasoning behind Clarity is a bit strange; "With no mana you can't go stealth or do dmg." The point isn't to spam your abilities; you should probably be harassing for quite a while in order to not mana starve yourself.

Regarding your mastery tree: Try putting 4 into Sorcery and 2 into Alacrity so you can get Archaic Knowledge; the magic pen helps greatly when you use Expunge.