Not Updated For Current Season
This build is based on Attack Speed in early-mid game, and then a balance of attack speed and damage at the end. Because the speed of your attacks is so high in the beginning, Expunge will be maxed out very early so you will get easy kills by quickly putting max stacks of your Deadly Venom on the opponent and Expunge when they start running. Later in the game, you will get items with damage, letting you wipe out your victims very quickly. Once you have your first 5 items (Doran's Shield, Berserker's Greaves, Last Whisper, Madred's Bloodrazor and The Black Cleaver), your best bet is to get the Sword of the Divine. There are several reasons for this. Number 1, it gives 55% attack speed, which is obivously a huge advantage and will let you put max 6 stacks of Deadly Venom on your victim in about 2-3 seconds. Also, it deals 100 Magic Damage every 4th attack, and at that speed, you will be doing an EXTRA 100 damage every 2 seconds on top of all your skills and godly damage. And lastly, Sword of the Divine's Active ability gives you Armor Penetration for 8 seconds, AND your attacks cannot be dodged. This item is essential against a tank with your Last Whisper (40% armor penetration).
Skills Combo for ambushing a single player: [Q], [R], [W], [E]
Firstly, make sure you are in Stealth and there is nobody else around. Then quickly turn on your ultimate ([R]) and the ability of Sword of the Divine if you have it. Then start attacking the opponent. By the time they start running, you should have all your stacks of Deadly Venom on them. At this point, you should use Debilitating Poison ([W]) to slow them down (if they start attacking you back instead of running, use Exhaust on them so they can't hit you for 3 seconds). When your slow wears off, Flash to them to get closer for several more hits. Once their HP is low or they are getting out of your range, use Expunge. Unless your opponent is a HUGE tank, they should be dead 100% at this point.
For team fights:
When a team fight engages, wait a few seconds behind your team while in Stealth (Ambush), and then do the same thing as you would if fighting 1 champion, but be more careful and get ready to Flash backwards if your team is losing the fight or you get pushed back. The trick is to line up your shots right with your ultimate so that it hits as many players as possible, so that when you use Expunge, you hit everyone.
I will add more info later, but I am a little tired right now. Good luck guys, and remember, when you're playing Twitch, you're NOT a tank and you do NOT solo 2+ enemies, that's not what you're meant for. Stay frosty.