Ok, so ive uppdated the build with a new/improved item order because of the lack of survivability in the previous items.

This is my first ever build on Mobafire,but i can tell you i have ALOT of experiense playing with Leblanc and i can say she is by far one of the best nukes in the game (if not the best!).
My build might require some skill so for beginners playing Leblanc or anyone whos playing her for the first time dont just play the first 10 minutes, say she sucks and leave the game.
Just play and learn from your mistakes.
I dont know why many say Leblanc is really hard to play , Personally i think she is quite easy once you get the hang of her combos, pros , cons (these will come later in the guide).

When i play Leblanc i usually solo mid because of her great harrasment abilities and massive damage output(+ more kills for Leblanc ofcourse ).
By lvling up Q+W you can easily harrass by just casting q, using w to flash there and get back quick by activating W again. ( more about what your skills do in the Combo & Teamplay section below).

Im absolutely not sayig she is bad at laning with a mate , actually she is a great combo with rammus , shen , sion and a few more because of their cc (taunts, stuns, slows etc...)

But soloing mid makes you lvl faster , get more money = 2x easier to gank with her.
There are ofcourse many advantages laning with others too like lets say you are laning with rammus , you have managed to kill your enemies and you are facing the turret when this occurs if you are soloing it will take a relatively long time to finish of a turret and usually takes many pushes to take one down but with rammus at your side you can easily take a turret down at lower lvls.

If you try this Build out and say: hey, this build sucks im all too squishy and die alot!
you are either: 1. stupid
2. you arent playing her right
3. not yet experienced enough (keep trying!)
In fact when i play Leblanc , depending on if im facing squishies or tanks , pros or noobs i can take another Rabadons deathcap instead of banshees veil for extra damage.
(the masteries ive chosen gets you a bit of armor , Mresi and dodge so you arent TOTALLY screwed if u get hit).

Try to gank as much as possible to get stacks to your mejai soulstealer fast for ability power mayhem. ( But dont leave your lane in trouble just for a gank!)

you can experiment with items according to your will and what enemies your facing but overall i think this is a really good choice but i guess you could switch rod of ages with archaengel staff if you like!Please Comment if you have good suggestions.

(i dont yet know how to put pictures of items in here , but when i do i will fix it, dont mock me ,_,)

Now when you face an enemy your normal combo would be E-Q-R-W but if your enemy is behind minions or just hard to hit just go Q-R-W and finish him off with E and ignite if possible .(after your Q-R-W he should be running so it should be easy to hit with E , if hes not running and has alot of health left you dont have good enough items yet to 1 combo, RUN and wait for your cooldowns to reset). Escape with your W and follow up with R wich should be the same spell if you dont use another inbetween.

Im now going to explain why yo use this combo for those who dont have the energy to think this one out or just doesnt want to read: your Q spell damages and lands a seal on the enemy wich can be activated by hitting him with another of your spells ( does not work with your auto attacks) the seal deals additional damage when activated + silencing them so using your
Q ability followed by your ultimate wich mimics the spell you used last and by doing this you will activate the previous seal for damage, silencing and putting on a even stronger seal wich you should / can activate by using W a flash that aoes the area u flash to and can be activated again in 3 seconds to the place you flashed from.

In teamfights your role is not to jump in first and dying as quick as possible ( as many noobs apparently think) but try to stay hidden, go nuke someone , hide again or helping a dying comrade. you have relatively long cooldowns on all your spells (exept Q) so you cant do much damage with your spells on cd so try to wait them out before engaging the enemy again, atleast wait for your W spell. ( for a shorter combo Q+W)

I take Flash as one of my spells because it can be the gateway to a succesfull gank by quickly positioning myself behind the enemy for some surprise butt *** :).

I also take Ignite wich can be good for low lvls when you probably wont 1 shot with your combo so it���´s a good way to finish someone off early game (and late game too ofcourse).

Pros
1.Really ,really great damage
2. she is good in all stages of the game
3.Escape artist, she can triple flash with her abilities + Flash, almost impossible to catch.
4.her passive makes the enemy confused if they get you low (makes a duplicate you and stealths you if u get under 40% hp).

Cons
1. easily killed if she gets a cc in her face
2.Fail 1 thing with your combo u have to run or die.
3.Targeted in teamfights , so stay at range and wait for ur moment!
4. only has 2 purchasable skins :S x)