Using the Ruby Crystal you should be able to get an early kill or two, which will help you get Phage. Once you have Phage, kills become considerably easier, and it just gets better from there. The Wit's end allows you to attack from two different types, aka magic and physical, so that no opponent will be able to mitigate you entirely. Once you've obtained the Bloodrazor, it's game over for pretty much anyone that crosses your path, as Bloodrazor at your 70% attack speed bonus destroys any enemy champion in seconds.

At this point as long as your allies aren't terrible and the enemy team is 4 VS 1'ing you(this HAS happened to me), the game should be in the bag, as you will annihilate towers easily as well. And sometimes when it's 4v1 and you activate your Ulti, you're gonna kill 2-3 and leave the rest near death. Using Ulti to go through all 4 + your AE poison damage is brutal, and can leave the enemy team complaining about how OP twitch is. If you're the one to catch them by surprise(Easy to do as Twitch) you may just kill them all at once.

For defense, don't forget to stealth while you are running away, if necessary, this can save your butt, especially if you use ghost first or duck into the bushes. Don't forget when returning to combat to stealth early once you have the higher ranks as it lasts a pretty long time(50 seconds)

Twitch is one of the most fun classes to play by far, his stealth plus attack speed is easily one of the best skills in the game.