Hi everyone, this is TheMese an op summoner at LoL :P, seriously this is my first guide and my english is not that well so keep that in mind! :D

Let's get to what we are here for:
Twitch is a ranged poison based champ which relies on positioning and doing tons of damage to an entire team, and/or sneaking behind enemy lines as an assasin, which certainly is not a recommended role anyways.

PROS:
-Stealthy
-Your lane will be 90% of times pushed, so less enemy ganks for you!
-W+attack+E+(ignite) can finish a low hp enemy in less than 1 second of hitting buttons
-Nobody plays the rat, so not everyone knows how does it works
-Is totally undervalued, some people think you are weak and do 0 damage
CONS:
-Very very very VEEERY weak, probably the same as Vayne
-0 escape tool,(except of Q which normally you will use to positioning, attack, or something before you realise you need it to escape)
-No gap closer, this isn't really a problem you have your Q and you're not an assasin
-Wards and oracles are your enemy, they don't couter you as much as akali, but you will hate them, believe me
-Squishy
-Weak
-Did I say squishy? And weak? Remember it, if you don't remember it you are dead

Everything told, armor penetration is also admitted, but you will lose some early damage and you want to dominate you early lane with damage and cs, so you can pass that stupidly bad mid game and continue to your awesome late game. If you reach late game fed, you will probable carry to win.

Normal masteries for an adc, except you want CDR for you mediocre high CD skills, moevements speed to reach faster your lane and the improved recall, so you can Q+B and go invisible to your base :D

Nothing more to say, you want to max E for the damage and W or Q after depending on your necesities, even some times you max your Q and W at the same time.
PD: Sometimes(a very few) you can skip leveling your ult at lvl 6, for mor harass without killing, so they wont believe you are getting fed, but warning, this is extremely dangerous and I only recommend you doing this if you are getting ahead at lvl, farm and kills and you know they are not going to attack you

This is were Twitch gets OP. Normally an adc will buy last whisperer at some point and a Phantom Dancer and that's ok. If you play Vayne and you buy a Hurricane you are losing att.speed, mov. speed and the critical chance, so you sell it and buy a Phantom, but if you are playing as Twitch you want to do tons of damage to the whole enemy team(EJEMpentaEJEM), so if you buy your super hurricane and your nasty Black Cleaver, pop your Q, wait the right moment and activate your ulti, you would stack Black CLeaver's passive to 3 enemies at least( hurricanes passive)+ everyone you hit when your bolts tresspass the enemy. Normally you would make the non-tank enemies have a negative armour value. So you must buy your hurricane. If you are doing preeeeetty well you can consider not buying your Guardian Angel and buying a Phantom, which would make you reach 2.5 att.speed when Q bonus is activated :D Want more reasons to love the rat?(Warning, buying this will make you hyper-susceptible to being focused so it's very risky).

There are two ways of starting your game, pots+boots or pots+long sword. By getting the sword you can recall and buy your vampiric scepter and your boots(maybe upgraded) but you will lose lots of mobility and early. Only make this choice if you know your suppoort can cover these lack os speed or you know you won't need that speed at early lvls. You should focus on damage because Twitch has one of the highest att.speed base in the game, so maybe 1 att.speed object would be sufficiente(you can buy 2 also to bonus from it very much!).

I wouldn't recommend taking ignite because your passive can kill a low hp enemy, or W+E an enemy out of range, but it is a great choice anyways. I prefer having a non-expected shield when I'm about to die, or taking out all the CC made to me :D I've seen many rats with Exaust and I don't say it's a bad choice, but your attack range is higher than Exaust range, so you put yourself in a risky position only to exaust him. If you want to slow them use your CC! So I only see viable Ignite/Barrier/Cleanse + Flash. NEVER take anything without flash, it is your only escaping tool, your Q will always be on CD because you will use it to figth not to escape, remeber that!

Not too much to say, Twitch is good at early game, but it doesn't farm well. It has low AD compared to other champs, but that is because you rely on your E to do heavy unsuspected damage, so most of the time you will be below your torret, which isn't really bad if you know how to kill your cs with your torret hitting them. Beware of that!

You don't have any specific adc counter, but you also don't counter any adc, but my experience tells that Cayt will make you have a hard time and Ez escapre your W and E rather easely.

Well, this is the end my friends, a pretty well done summary would cover everything without having to read the previous info:

Twitch is a mobile champion that requires high skills of adc by knowing when to attack and WHERE to attack from. You are squishy as hell so rely on your teammates to cover you. You don't having escaping tools except of your flash, remember that. Always farm, ALWAYS, even if you are walking to change lanes or to go mid, try to kill some jungle creeps.

I hope this guide helps you a lot and please if you need any type of help contact me at EUW lol account (themese). Thanks for reading and feel free to comment and support!