Early game just focus on farming creeps with last hit, that gives you a lot of money and experience. Twitch is great for mid. Vampiric Scepter is important item at the beginning because if you kill someone or your HP is low, so you can grow your HP faster. Ambush spell is great for ganking because no one can see you and your kill/assist is sure.

Twitch is team fighter. You need to wait the tank to go first and then, if your ultimate is ready your position need to be ready for piercing enemies. You need to use Venom Cask first when your target get 6 stack then use Expunge, because if you use Expunge first stacks will be lost and your target will be slowed for less movement speed without 6 stacks.

- you need to slow your enemy before he run away.
- you gotta escape with low HP if they are following or targeting you.
- Additional poison, but i think it's better to get Flash because you are slow and escaping with low HP, then using Q spell, so how you want:)

This build isn't crit, so i'm not getting Lethality and Deadliness . Twitch in early game have little mana problems and i'm going on 9 points to Utility for small mana and mana regen.

Greater Mark of Desolation is important, because you will do more damage to enemy units and minions too.
Greater Seal of Health. Twitch has low hp, he need and in my build have a bit life steal for surviving after fights with killing minions or jungling...
Greater Quintessence of Health. -||-
Greater Glyph of Attack Speed. attack speed for stacking poison and, of course you are finishing him with Expunge to save your kill or if you are EVIL you will only kill steal.