Welcome to my first guide ever :D

To get started, LeBlanc is probably one of the best early/mid game champions in the game, but she is really bad in the late game team fights, constantly getting smashed and focused by the whole enemy team, but more on that later.

The regular 21/0/9 Masteries. These are essential as she needs AP and Mana Regen early game as she tends to go through it fairly fast. Spell Vamp isn't important enough since you get Will of the Ancients.

These runes are built for early game Mana and late game AP. You could substitute the Mana Runes for Magic Penetration Runes for that extra late game boost.

Definetly start off with a Doran's Ring as this give HP, Mana Regen, and AP. First tier boots are next so you can catch up to your enemies that are E'd easier. Next, another Doran's, or you could start building your Mejai's Soulstealer now because getting to 10 stacks is mandatory by level 11, but its all situational, if your lane partner isn't the best and you have died a couple times, then buy a Doran's Ring. Sorcerer's Boots are next for that Magic Penetration and Speed. Rabadon's is the next item to build, you'll probably have it by level 11/12 if you're good at last hitting and have at least 5 kills unless you're mid. Will of the Ancients, an important part since you will be nuking the f*ck out of your enemies, this will increase your survivability by a lot. Zhonya's Hourglass a great item for when you're using your E and other enemies are around or when you're getting ganked, also it gives you armor against those pesky carries. Guardian Angel is a very good item too as it gives you more armor and magic resist and it has a great passive to allow you to potentially save those precious stacks late game.

To effectively nuke you need to have the right skill combo, I usually go E, Q, W, Ignite, R, Q and then another E if they're getting away... But usually they're are dead :D but if they still aren't dead by then, just give up lol. During the E you can cast spells without interrupting it so if you time it right you could kill them once your E ends and stuns them. Also during the E follow their movement, if they're running away follow, if they're coming towards you run away. This is just my combo you can find other one that you like and goes more with your game play style, but in order to nuke this is the best way.

Pros: VERY VERY good early game able to kill Champions easily at level 4. Great for fast combo nuking.

Cons: Squishy and easily targetable in team fights so its best to stay in the shadows till you're teammates and enemies are low HP that way its easy pickings :D also a note stay away from most carries because even if you're fed and have 20 stacks, a decent Tryndamere could come along and f*ck you up easily.

I haven't been playing for that long, so I don't know all the champions that are good against LeBlanc, but I know carries will mess her up and also Champions with slows will also. That being said Tryndamere is a very good counter to Leblanc as he is both a carry and has a slow.