Ok so... first thing i want to say is that this IS my first build i ever made, so please dont hate cause im bad at this. Second is that this build is not full AP but more of a hybrid type of build... as you can see there is the guinsoo's and the hextech wich gives AD as well as AP, AS, Spell vamp, and Life steal :D.

so here it goes....

Pros:

Alot of burst with expunge+AP
still have good AD dps
VERY good multi killer in team fight (ulti)
can stealth so u can sneak up on people and kill em :)
can finish off kills that run with his passive

Cons:

Very squishy

VERY vulnerable to CC
stealth ability could be easily dealt with, with an oracle elixer (but i got a some what of a solution ;)

Masteries
now i go with a 21/0/9 mastery tree cause, you would want lots of damage while still having some utility like increase gold and exp... even if it is just a little bit it is still worth it.

HEALTH HEALTH HEALTH!!!

as twitch has VERY little flat health (both beginning and at late game) u NEEEEEED these runes, OK maybe not need but since they are flat health runes it works very well with the beginning of the game :D. i cant really see any other runes benefiting him as much as these, if u think there are better runes plz comment and i will add and stuff :D.

BURST

Early Game

ok so now here... i go for stealth first although you wont benefet from the AP u just bought, you are going to have to get it. the point is, its the only way to get up to start shooting at an enemy with your crossbow at lvl 1, u shouldnt try that though it will end up with u getting hit alot while not doing so much damage your slef. Second spell to get is your expunge now, try to keep out of trouble as in dont try to kill a person or gank with your lanning partner before lvl 5, because as long as you follow my skill sequence you should have upgraded your expunge up to three at lvl 5 this is where you start with your burst your expunge is your MAIN burst. now once you get guinsoo's your in mid game.

Mid Game

Now this is where you shine... with your burst and stealth you can finish off people in seconds. and at level 8 when you get your slow, you will really start to get kills without getting killed much. it easy just stealth, pick an enemy champ, tell at least one of your team mates to came help you kill him. when you encounter the enemy, first let your teammate go in because you cant take damage (VERY squishy) you should pop out from behind the enemy, for him to feel trapped >:D, when he tries to kill you or is running or going away, etc... pop your slow, with the slow it wont matter how much stacks of poison the enemy has as long as he is slowed and there 2 people against him he should die quick. It is BEST that you cast expunge with full stacks of your poison on the target (or when you have your ulti and it finished.), this will do massive burst plus 100% of your AP so you will do lots of damage in just one spell and its range is Very big so dont worry if he tries to run with low health on him. But careful, as you poison stacks are consumed when you use expunge

Late Game

Here you would start to fear going in and killing much, in late Game you MUST stay back (behind your tanks and teammates and once your in team fights pop out of nowhere (stealth) pop your ulti hit them 7 times and expunge (you can slow but is just wastes poison stacks and time its best u do it when they run away). i cant guarantee a penta kill(still possible), but atleast you will get a quadra just doing this.

AP+AD=CRAZY BURST+DPS

OK so now i start with an doran's ring in the beginning (for the health and the mana regen, believe me you need the mana regen, plus the AP too).

i then get a Kage's lucky pick which gives a bonus 5 gold per 10 secs (it will pay it's self off and give you more money as you keep it.)

then go for boots of Mobility, as you really want them for your stealth (cause you wont be in combat while stealthed.)

after that buy another Kage's lucky pick, those 2 will give 10 gold per second together plus 1 from the mastries and normal that is 21 gold per 10 secs which is 2 gold per second.

after the pick i grab a Guinsoo's, WHY, A GUINSOO'S? u ask... its simple first off it gives a decent amount of AD and AP just from its basic specs, then the passive: each hit or spell cast gives a stack of guinsoo's which increases AP and AS, as you need 6 stacks of poison on an enemy to get expunge to do max burst, this is like an added affect to that so the more you hit with physical attack you apply poison on the enemy and you get a guinsoo's stack which both benefit for your expunge.

After that i get a hextech, this gives twitch another slow, AD, AP, and spell vamp and life steal for survivability (in team fight if you use your ulti and use expunge you can get your full health back, even more!), so if you are in a team fight and you wanna stay up and alive for your team all you need is your ulti and expunge, you will get so much health back, also while getting hextech you may sell ONLY ONE of the Kage's lucky pick, as you need the other one.

after hextech i get a rabdon's deathcap for the holy BURST, this itself will get you to do 201 more burst with your expunge (without full stacks of poison) ya this is pentakill material were talking about here ( if you follow my build up to the deathcap you should be able to do about 1250 damage to people with full poison stacks, using expunge of course).
After that i get a death fire grasp as it gives me AP, CD reduction and upgrades from lucky pick and its active is a VERY good beginning burst (wastes 33% of targets current health!!~!!.)

After that i finish it off with a nashors tooth for the AS and AP and cooldown reduction :D.

Note: im actually STILL testing and trying stuff out so plz comment and give suggestions on the items and the other stuff :), Also the AP at level 18 says its like 450 AP that isnt true because of deathcap's passive and guinsoo's passive they are WAY more, like 600~ AP

Wards...

OK... so now you want to try out my build and find out that the enemy team is smart enough to put up some wards Early game. they will most likely put them in the lane you are in, happened many times with me, there is a fairly easy way to counter it. if you have a an able team mate then switch lanes with him. Example: im lanning with a blitzcrank in bottom lane, i notice that the enemy can attack me even though im stealthed and they don't have oracles. What you do is just ask for a lane switch with top lane, so blitzcrank stays in bot with your other team mate, and the enemy team just wasted gold :). note that the Wards only stay for 3 mins, if you count it you can switch back or gank with the lane they warded in 3 mins but i choose to w8 a bit longer.

Oracles

Now this Also requires able allies because if you notice an enemy with oracles, the ONLY way to get rid of it is to get rid of HIM. So ask your team or a couple of your team mates or at least 1 team mate to gank him and finish him off. after that he should be too scared to get another oracles and he just wasted gold :D.

i hope you liked it and tried it out i worked hours on posting the build and days of testing and i will put up screen shots later... and remeber

BURST

:D

NOTE: im still adding more and more chapters and info and i will be more than glad to take your suggestions. Chapters im adding later: Other situational items & and i will think of some later; also add me in LoL US my name: moaz