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 is my first attempt in publishing my build. I've read through a few Twitch builds and thought I'd post up my own. Any suggestions / criticisms are welcome.
~Vampric Scepter: Twitch is one of the weakest (defensively) character in this game. In early stage, taking some damage is just unavoidable sometimes. When I'm in a lane, I try to replenish my HP by life stealing minions if I were to take some hits from enemy champions. This ensure my survivability to gain gold and experience quickly at the start.
~Malady: Finish off building on your Vampric Scepter by purchasing Maladay whenever you get a chance. This increases your attack speed by 50%. Assuming you have 1 or 2 levels of Ambush at this point, you'll find the burst damage under Ambush's attack speed bonus to be quite formidable in early
~Berserker Greaves: Time to enhance movement speed. I chose Berserker's Greaves because it has the attack speed bonus on it, which helps even more with Malady and Ambush combined
Last Whisper: Getting Last Whisper at this point, when I'm assuming you're at a level with about 3 points in Ambush. With your attack speed at this point, you should be able to push close to 2 attack speed at this point. Having armor penetration increases your damage, and your quick attack will be able to apply your poison on your opponent quickly for Expunge
Infinite Edge: Now it's time to build damage as your attack speed should be set until the end of game. Infinite Edge is my top choice, and Black Cleaver would be my optional. With this your auto attack should be hitting much harder.
Running this build, as you can see, I try to max out Expunge first. With your items stacking attack speed as your priority, you can quickly put up 6 stacks of poison on your opponent without relying on Ambush. In ~3 seconds of time, you would have 6 stacks of poison on your opponent. At this point, I would cast a Debilitating Poison (In case your opponent decides to start running) and Expunge. This way, even if Expunge does finish your opponent, you can still catch up and do a few more attacks. If it is a group vs group situation. I would stay cloaked until the opponents' attention are on my teammates, then pop Spray and do my normal rotation to knock them all out.
I have Cleanse to prevent any sort of blind or stuns which will reduce my burst damage. Flash is for me to have my get away. In a tough situation, I'd flash away and quickly go into stealth to avoid chase from my opponent.
I basically try to focus on the strongest advantage of Twitch and further enhance it as early as possible - his attack speed. In my build, by the time you finished purchasing Maladay & Last Whisper, and when your Ambush is at lv 5, your attack speed should be at about 2.3, which makes your autoattack deadly. Topping that off with 6 stack poison expunge can quickly destroy your enemies.