THIS BUILD WILL BE LAUGHED AT, BROUGHT DOWN AND WILL PROBABLY NOT WORK!
Use it for fun only! It CAN be better for teamfights than the traditional twitch but dont count on it.
This build will make you do mass amounts of damage with your expunge without the need of multiple stacks of your poison. You should be able to hit your target 2-4 times, than kill them with expunge. This will be unexpected so tanks will attempt to counter you with armor, or people with exhaust will think they have you. When people see how low your hitting they will get closer to you, thinking they can kill you before you kill them than, BAM, over 1k damage in their face!
This will counter things such as exhaust as listed before and teemo's blinding dart.
The game will start out as any normal twitch game, stealth in the bushes to see if there's anyone in there, and try to kill. AT level 2 you should be trying to get random shots off at your targets, if you get over 3 stacks its time to expunge. It should be doing pretty decent damage, even at this level. Without the runes for AP it deals 119 damage with 6 stacks. You can surprise someone with around 200 or so health by eating it all away in seconds.
Once you get your mejai's, its time to gank. Like any other twitch you look for someone that is closer to your turret then they are to there's. Stealth up behind them, unleash your auto attacks, slow them and nuke them with expunge at 6 stacks. If done frequently and correctly you should net quite a few kills and be on your way to getting full mejai's stacks. This is very important as a full mejai's plus your Nashor's will get you full CD reduction. Nashor's gives you everything you want, AP, CD reduction and all the attack speed you will need. With a rank 5 stealth and your ult you can reach 2.0-2.5 attack speed.
Zhonya's is quite obvious as it gives the 25% boost in AP, and the effect can save your ***. Archangels is mostly for the boost in mana, as you do not have too much, this and Void Staff can be replaced early on with a sword of the occult, if you really want the extra attack/movement. Boots CAN be switched with others but i strongly suggest you get the magic pen ones, as it greatly increases the damage of expunge.
Stealth up, target their most important player, such as a powerful dps champ like tristana, start attacking, activate your ult and when you tag a few targets with your passive, at least 3 stacks, use expunge to wreck their chances at winning the fight. You are about strong bursts, use expunge whenever you can in teamfights, with max CD reduction you can do it very often. Spam it, abuse it, laugh about it.
All in all this is more of a joke. They wont expect it and may not be able to counter it if they already built for a dps twitch. Do NOT use this if your team is all magic damage, only use it if you say need a mage or if your with people you know. Try it out in a practice game, see what you think, feel free to change the items up to the ones that suit you. Want more health? Get a rylais! Need more magic pen? Haunting guise! These are just what i found to be the best for this build. If you lose with this, it's not my fault. I warned you. If you win, good job! You pulled off a very fun, different build that no one would ever see coming.
Of course, enjoy yourself! If this is your style of odd playing than do it! It's new, it's fun, give it a try and if you don't like it, than just go the same old dps twitch. Very, very, VERY boring twitch.