Welcome to EndGameVayne's LeBlanc build!
This is my first build on MobaFire, so please rate it.. And don't be really harsh to me :)
LeBlanc is a pretty low ranged, nuking, mage mid/support champion. Lets take a look on her abilities:
Passive:Mirror image.
When LeBlanc is brought below 40% health, she instantly becomes stealthed for half a second. When the stealth fades, she creates a Mirror Image of herself that can move, deals no attack damage, has 53 + (17 x level) less health, and lasts for up to 8 seconds. This can only occur once per minute, regardless of cooldown reduction. The illusion's autoattacks will proc on-hit effects (including damage).
This will be used same way as Shaco's hallucinate, without the damage - dealing explosion. Basically, it will be pretty useless early game, but it will help You to confuse enemies late game if used properly. Today it got kinda buffed - now clone will run and wont be standing still - confuse enemies even more? GREAT SUCCES!

Q:
Sigil of silence.
LeBlanc projects an orb towards her target, dealing 55 / 80 / 105 / 130 / 155 (+40% of ability power) magic damage and marking the target for 3.5 seconds. If the target takes damage from one of Leblanc's abilities, the mark will trigger, dealing 55 / 80 / 105 / 130 / 155 (+40% of ability power) magic damage and silencing the target for 1.5 seconds.
Well THIS IS BEAST! Alot of people says, that it ust got nerfed - like "now you have to trigger it, FAIl; "no more free q harass, FAIL", etc. I dont understand, ARE THEY SERIOUS? Now like at level two, let's imagine you have 35 ap with your runes/masteries. Enemie's max hp is 600. You drop Q, deal about 70 damage, then W for 70 more + 65 damage from W = About 200 FREE damage ( calculated with magic resist they have, around 30 of it.) Then W back (cuz u have all of cooldows to refresh) - Wuolah! 1/3 of enemy hp for pretty much nothing!

W:
Distortion
LeBlanc rapidly moves to a target location, dealing 85 / 125 / 165 / 205 / 245 (+60% of ability power) magic damage to nearby units. In the following 3 seconds, she can activate Distortion again to return to her starting location.
Aaaand another godlike ability! It is REALLY good to juke - drop W over a wall, enemy flash after You, switch with W - You escaped, enemy flash wasted. The only AoE ability on LB. Dont use it to engage a teamfight, cuz' you wont be able to disengage later and You'll have to flash/ult.

E:
Etheareal chains
LeBlanc flings illusionary chains towards a target location. If it hits an enemy unit, it will deal 40 / 65 / 90 / 115 / 140 (+50% of ability power) initial magic damage and slow their movement speed by 25%. If the target remains shackled for 1.5 seconds, the target takes 40 / 65 / 90 / 115 / 140 (+50% of ability power) additional magic damage and is unable to move for 1.5 seconds.
Not a really good one, I usually use it to catch an enemy or trigger sigil of silence mark. Actually it's good to trigger sigil of silence, bcz' it will slow, silence (so enemy cant interrupt the chain) and snare later. An average ability.

R:
Mimic
LeBlanc can cast the previous spell she cast.

Mimic: Sigil of Silence: Deals 100 / 200 / 300 (+65% of ability power) damage once on impact and again if the mark is detonated.

Mimic: Distortion: Deals 150 / 300 / 450 (+97.5% of ability power) damage on impact.

Mimic: Ethereal Chains: Deals 100 / 200 / 300 (+65% of ability power) damage once on impact and again if the target is rooted.
WOAH!!! THIS IS LEBLANC! Just look at the AP scalings! Low cooldown! More damage!
Beast ability, You can Q to insta R for half of enemy hp, then W to trigger R's mark to finish him for fatality.
It has almost 1 AP Scaling with W and another mobile ability!
E+R = Duoble snare! Stronk!

Welcome to EndGameVayne's LeBlanc build!

LeBlanc was one of the first my ACTUAL mid lane champ. I remember that I saw TheOddOne playing as LeBlanc, bursting adc down with 2 abilities, juking like a boss, and i was like "Hey, imma try that too!". I went to PBE, played her for the first time, ended up like 21-9-5 or smth, and then I realised - She's a champ for me! AP, which I like more, kinda trolly, damage dealing,has decent CC, damn hot.
And now You are here, reading my build to her. Hope You will enjoy it! :)

Pros:
One of the best bursts in the game,
2 snares and 3 silences,
THE JUKEES,
Hardly gankable,
Really strong mid-late game,
Still can blow almost anyone 1v1 even if not fed,
2x1 IF YOU KNOW WHAT I MEAN

Cons:
Mana hungry,
Shouldnt be played without runes/masteries due to her weak late game,
Easy to learn, hard to master,
You can possibly troll yourself w/ your W,
Passive useless early game,
Short ranged.

RyzeDifficulty 5/10
Ryze is a decent match-up to LeBlanc. Ryze and Leblanc are both pretty short-ranged, Ryze has to walk up to You, but if you get snared, You cant use W which is pretty bad.. But in this fight, the better would win!

Galio:
Difficulty 8/10
Ohhhh my god just not mid Galio. He will CC you, then turn your burst damage (which wouldnt be high cuz' of his MR) into health, then CC you even more, ignite and kill. Just a really annoying mid against every AP champ, even tho he's underused, I don't know why. And remember, DONT USE YOUR E AGAINST HIM HE COUNTERS ALL OF DoT's!!!

Ahri:
Difficulty: 2/10
See Ahri as AP mid in enemy team? TAKE LB! Her main damage/cc is skillshot, You use W twice and she will waste half of her mana to nothing. She ults on You? E is your friend. Whe will use ult to get out of your E or she will get snared. She's kinda squishy tho, so you will burst her w/ Q-R-W. Just an easy one to kill.

Akali:
Difficulty: 4/10
Akali is a strong champion overall, but champs with better early game then her's can crush her. I just had a ranked game against Akali, just spam Q-W on her, when she's around half hp, ask jungler for help, get a few kills on her and then snowball hard (which LB is good on) and You will win the lane!

Fizz:
Difficulty: 4/10
Everyone says, how hard it is to go LB against Fizz. Say what? Your W should be off cooldown more usually then Fizz's E, You can W-R if he ults on You so he wont get a kill. Well, Fizz is a strong champ tho, so a better player will win then lane.

Karthus:
Difficulty: 5/10
This guy is pretty tough, he can farm and harass w/ his Q at the same moment. Just try to overfarm, then B with about 60 farm ( You should have ~2000 gold atm), Buy negatron, Blasting wand, then go all on him. But dont do it if You know that you will be left less then 150 hp and he has ult up. Then JUST DONT.

Ziggs:
Difficulty: 1/10
Such and easy lane! Just ZONE him, so he cant farm. You REALLY outdamage him, mid game you can one shot him. Hes long range, and all he will do is harass You with his Q, which is not that hard to dodge. So just go all agressive on him, start with item like rabadon.

Huekaiser:
Difficulty 8/10
He's a tanky damage dealer. Try to zone him early game, till he's not tanky, cuz late game your abilities will only scratch him. Outfarm him, and if you do, go agressive. If his farm is bigger, and he's zoning you, ask junglers help and buy defensive items. Morde will push the lane like mad, then You will have a big trouble lasthitting under tower. Any champion who specializes in single target burst, can't last hit under tower, or can't deal with pushes will have a terrible time vs. Morde. I.e. he is very strong against assassins. Once Morde gets his shield charged, LB is just going to waste her time harassing and end up going oom. I face an LB as Mordekaiser before, just hid behind minions, she missed her skillshots and her QW didn't do much at all. At the end of the game, she ended up with 42 cs where I had 172. If you are gonna play an assassin vs. Morde, you are better off just letting Mordekaiser push you for free while you roam or farm jungle. So just dont take LB against Morde, and if HE counterpicks You, just try to swith lanes w/ Your solo top. And remember, WHAT TO DO IF YOUR LANE IS GETTING PUSHED?
THATS RIGHT, CALL THE JUNGLER!

Orianna:
Difficulty: 7/10

There is no real counters for a good Orriana. Yes, You can Juke her skillshots, but think about a GOOD Orianna. She would understand that she cant 1v1 You, ask jungler/solo top for help. And Orianna in teamfights is like 3 LeBlancs. You can go agressive on her, but remember that She is the champ that You should really get lot of wards against. GOOD Orianna will build some MR against You (Banshee, Chalice of harmony). So just go agressive very early, and dont fall behind late game, which Orianna shines in.

Sion:
Difficulty 7/10

Another Mordekaiser! He has shield, so You wont "Blow him up", he will push the lane, You will have to lasthit under turret, lose money, lose xp, lose lane...
So what to do?
Early game your aim as leblanc should be to nuke his shield with a Q+E as he comes to you, if his shield pops your free to go all out. If you manage to get 2-3 kills ahead by mid game you should be able to snowball by ganking other lanes and get ahead.
And dont forget... It's a hard lane sooo.. JUNGLER PLS HELP ME MID THIS SION PUSHES ALOT AND HES EASY TO GANK!
Srsly. Well ganked Sion = Dead Sion.

Thanks EVERYONE for reading, please rate, comment, let me know what I did good and what I did bad. After all, LeBlanc is a REALLY good Mid/Roaming/Focusing important targets champion, and she's countered by pushing lane too much. This have been EndGameVayne, see you lator! :)