First off, Leblanc is insane IF you can handle the powers right.
The main thing you will hear when you pick LeBlanc is "lategame sucks!" and that is true in a certain way.
There is no way you will be able to burst down a endgame tank, and yes LeBlanc is a burst champion but the ad carry.. Oh he will not live.
A quick note on the masteries, theese are what I use for some extra health and that 10+ tower damage is actually pretty good if you think of how much damage it's otherwise.
Q,W - This combo is extremely effective at level 2-5 where basically if you hit it right, you will win every single trade mid.
W,Q - This is a safer and easier thatn the first one that basically a poke, you do small damage earlygame with this but probably wont take damage since it can be done from a long range.
E - You E spell is a skillshot that does initiall damage and then after a short wait binds the target and damages more, if you do the E after one of the earlier combos you can use Q again after about 0,2 seconds of the bind.
R - LeBlancs ultimate casts the latest spell again but with increased damage, since your Q is the main damage dealer in most cases hit a Q, R combo, so you just add R after the Q on the other combos. It can also have a huge utility value since you can have two W spells with this ultimate, you can get extremly far if you first W and then R towards the same direction and you can get away from ganks etc.
Poking is basically just doing a harass, a small combo to bring their health down before a burst. Remember to have enough mana to burst. If you have blue buff, poke as much as possible.
How to play vs different champions
Heimerdinger - Try to avoid his turrets when you burst and attack after he has shot is two other abilities and stay out pretty far back so he can't harrass. But still remember to farm.
Morderkaiser - Morder is one of the hardest to play LeBlanc against just for the tankyness he has. Don't do your burst when he has his shield up, poke it down and dont let him take the wraiths camps (as many morderkaiser players do). Try to let him hold the push as he usually does and ask your jungler for an early gank then you will need to take care of your advantage.
Twisted fate - Tf is a relativly easy mid champ to play agains as LeBlanc, don't attack when he has his stun card up and you should do just fine.
Viktor - Now the tactic versus Viktor is similar to the one used vs Twisted fate, Avoid his laser and do not stand in the stun circle (this is easy to dodge with the W spell.
Ziggs - Try to poke as much as you can until he uses his minefield, that's your go. When he has used the minefield you do your burst and he will probably not survive it.
Galio - Ask for a lane switch, you can play bot as support or top (top is better) just switch with some AD cause Galio is to strong vs AP
Lux - Avoid her bind and after she has used that ability you can freely burst, also if possible try to make her pop her shield with a poke before you going to burst.
Morgana - Morgana is practicaly identical to Lux thus you should use the same tactic, make her use the bind and shield before you burst.
Ahri - Ahri has a very powerful ultimate vs leBlanc for that you should not burst whilst she has it unless your certain of a kill, for if not she will kill you. Avoid he charm and since she oftens has push on her lane ask your jungler for an early gank, this way you should have your ultimate before she does.
Fizz - Fizz is also very hard to play versus as LeBlanc, you cannot burst him down cause he will simply do his dodge ability, thats why you must trick him into doing it earlier by poking him alot, be cautious of his ultimate.
Kassadin - Kassadin has a powerful silence ability that you must avoid, its basically the person who fastest can get there silence on the other player that wins the trade, for this you must be quick with the Q, W combo.
Katarina - Save your W ability for when she does ulti to simply counter it, after that its free go for the kill with a burst. Poke alot then finish with a burst.
Anivia - Avoid her stun, if you get hit you need to go out of her range for she will do insane damage, when she has used her stun and missed it you can burst her down.
Annie - Annie has alot of abilites that makes her capable of bursting and stunning. Make sure she doesn't have her stun up when your going to burst her, and also remember to be afraid of her bear, it will kill you. You don't need to poke that much early game since she is pretty weak.
Brand - As brand has a skillshot stun you really need to avoid it, if he does not hit his stun it is almost always LeBlanc that wins the trades, for that you need to poke out the stun before you burst. This is also a valid situation to ask for an early gank.
Cassiopeia - Hard to poke for she has so much harass abilities, also her ultimate is very good to stop the burst from LeBlanc, for that you should not attack her whilst she has ultimate. This is also a case in which I would recommend you asking for an early gank to get the ultimate advantage.
Karthus - This is not as hard as the others due to her ultimate not having effect on your burst. Stay away from her poke and harass and then burst her down, she is very weak so it should be no problem. Remember to leave her area when she is dead.
Kennen - Watch out for his Q cause it does damage and gives him the ability to easy initiate with a stun, when he has missed that or just used someother CD you can burst but I would recommend some pokin first to make sure you can do alot of damage if not kill him.
Malzahar - Stay back, it's hard to win trades versus a good malzahar, this is another "Battle of the silence" try to get your stun up fast when you poke/burst. And again it's a valid case to call for an early gank for the power of his ultimate is over 9000.
Orianna - Avoid her ball and poke her until she uses her shield then burst her down, when she has ultimate make sure not to get caught in it.
Swain - Swain is very good at trades so you should avoid getting in range of his spells. Make sure to get the silence up fast and if there are no minions close to him and he uses his ultimate try not to make him gain health by staying out of range, this way he will loose mana fast. Also if he recently turned his ultimate off it's a great opportunity to burst.
Ryze - Ryze is hard to play against as LeBlanc for he has great burst, bind and sustain, avoid getting low hp and try to poke him. When you burst make sure to hit the silence as fast as possible to make sure you will win the fight.
Veigar - Try to poke enough for him not be able to farm up AP with his Q, with this in mind when he has used Q to last hit you can burst him down without him being able to do to much damage. Buying abyssal scepter is a valid option since his ultimate uses 80% of your ap as damage, be aware of his ultimate doing loads of damage.
Xerath - dodge his abilities, specially when you have his stun on you. after that, burst him down if he still lives make sure you don't get hit by any spells since they are relatively easy to dodge.
Mejai's soulstealer is insane if you get up to the 20 stacks, however if it does not go your way I suggest selling it skipping to next item in the build and adding a new item like abyssal scepter etc.
Team fights as LeBlanc are relatively easy to understand.
Kill the carry, unless he has bought ****load of MR you will destroy him probably, same goes with support, do not stand in melee range of any melee, same goes with ranged, use your W to easy go in and out of fight. Once you have zhonya's you can be more central in the fight.
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