Twitch is a champion in which you get first hit easily and able to manipulate poison to your advantage. As twitch i have made a crit as build which deals massive damage, from personal experience you should be able to get first blood most of the time. So far since i have claimed mid practically every game for twitch i ALWAYS got first blood.
To be a good twitch you need to be able to make them think they are winning numerous times I have lead my opponent on while losing about 200 health then killing with expunge. By doing this you get even more fedder because they stay in more battles. Eventually they catch on so you will need to be smarter, you need to position yourself correctly, and determine what abilities to use in what moments.

For runes i like to go attack speed and armor penetration for that first easy kill. By having the armor penetration i penetrate all their armor at level 1 in most cases. Also, by having increased attack speed( about 0.15) it helps me deal more damage because I get to distribute more attacks to my enemy. Some say go flat hp seals which is not viable for twitch i prefer the extra as over some extra health, I'd rather have more damage output instead of being a more tanky squishy ad carry :)

For masteries you put into offense and defense even though i said i rather not be a tanky squishy guy there are no other viable options instead of increasing haalth and base defense. This will help you now and later when you get that +100 hp which may save your life in some cases

For items i start out with a dorans blade for the health and damage. It helps in the first few lvls but is kind of useless later on unless u have a free slot and impatient for the higher-priced items. then i move onto boots which are only temporary actually, late game i sell them for that last madreds to increase my damage output even greater since the only benefit of the boots would be speed it wont be neccessary since i wont run because of my amazing damage+ if i need to i have stealth flash and a slow. then a vamp scepter for some lane sustainability. After this get a infinity edge for some damage and crit % / damage. First, you get a b.f sword or pickaxe then the cloak of agility for some increased damage. After this get a Phantom dancer to compliment your infinity edge for increased crit chance and increase damage output by alot thanks to the attack speed. After a PD you get a bloodthirster for some more sustainability and damage. After the bt get ANOTHER infinity edge people question this, but it is good even though the passive doesn't stack. The attack damage AND crit chance increases your damage output and consistency by a lot. After this get yet another infinity edge for the same reason now you have 100% crit chance and should do around 1k+ damage. Now you sell your boots, after you sell the boots get a recurve bow for that attack speed back because you will need it for increased damage output then complete the madreds for your final build item. The endbuild damage can range from about 1k-1.6k depending on if a tank is going full out hp without that much armor for that massive 1.6k.

If your opponent has a thonrmail instead of a 3rd infinity edge get a 2nd bloodthirster instead for more lifesteal then tm damage

For skill sequence you want to start out with ambush for that increased attack speed and first hit so u can get the first blood. after this you max out expunge and if u can't put any into expunge put into ambush. Also, the reason you get your w so late(lv 7) is because you wont need that slow until it can compliment ur ultimate for increased accuracy from ultimate and helping the enemy not get away.

For summoner spells i generally go for ignite and flash. Using ur Q(ambush) then flash will probably make u invisible because itll keep u away from enemies for a short time which you need to become invisible. The ignite is for increased damage and killing when running away. For example, i have had many case scenarios where the person leaves at around 50 hp for me not to get first blood however they have my ignite on them so i can kill them and i gain the victory.

some Pros for this champion are:
You usually get first hit

You are able to roam freely

You can escape fairly easy

Eventually your opponents wont be able to escape

MASSIVE DAMAGE AND ATTACK SPEED


Some cons of this champion are:
Is fairly squishy

cant do well against tanks early on

can be countered by a shen or rammus really easy(thornmail/taunt)

if targeted while not having ult in teamfight you are screwed(the ult is actually good and will help with multi-kills(double,triple,quadra,penta) if you do not have it and you are doing wwell they will most likely target you and you will probably die

if countered right away (vision ward oracle) may not do well late-game(early on is really early considering that you dont even have the IE yet)

I generally go aggressive 90% of the time, but if one of the cons is in play(countered right away) you should play more passively only getting minion kills instead of champion kills.

To be an aggressive twitch and succeed you will need to get behind the enemy to make sure the escape route is more narrow considering that you will do more damage and they will more likely die. Also instead of using expunge when you have 6 stacks use it where appropiate(based on hp) even though more damage is better you dont need excessive damage, for example i camped out invis for a few seconds til an ahri at 400 hp came up i attacked 2 times then used expunge she died right away.

Also, like i said in the example camp out near the turret line, if you have a decently high ambush(lv 2-5) they will wait around 10 seconds come and then u kill them. This works especially well if you are low health as well for example 200 health and opponent is 250 health. They will recklessly come forth and you will take them out fairly easy.

When to go out for first blood depends on opposing team usually they have a jungler so wait until the 2nd minute use ambush and get behind the enemy because they will most likely be there within the 10 second time-limit for ambush. Once you get behind attack them and when they are at half hp use the ignite for the secured kill, usually people dont run away right away for example when full-health so you can use that to your advantage because usually they think twitches are weak which you actually arent and you will just astonish them with ur output damage.

When to attack: if you have your slow already and an opponent is low hp, but with a partner wait until they are a decent distance away from their partner attack them get the kill slow the partner then run away using ambush as soon as u can. If the opponent is at the same hp as you you will most likely get unless it is a tank/ le blanc(op burster --') attack them from behind and just dominate. Though, before you do this check the side brushes(assuming you are mid ) for any gankers. Also, if your teammates ping for ur help try to gank as many times as u can for feeding mid wont generally call mia too quick for them tonotice.

When an opponent gets a ward get an oracles right away, if they get an oracles have a teammate gank or come around and attack from behind to counter those advantages.

This is my first build i know the presentation isnt that well, but give constructive criticism and rate after you try it out. I hope you do well with my build

NOTE: I know this is not good enough presentation, but I'll completely finish this within the week it might take awhile(probably finish on the weekend) because of things goin on with school (graduating 8th grade ^^)