Hello there;) LeBlanc is an awesome champion:) She is good and VERY fun to play. For all who haven´t tried her or got her, You should relly give her a try!

In Marks take Greater Mark of Magic Penetration for more damage.

In Seals take Greater Seal of Replenishment

In Glyphs take Greater Glyph of Scaling Magic Resist

In Quints take Greater Quintessence of Ability Power

Those masteries give you better damage and some utility so you can stay in lane without going back.

Mejai's Soulstealer is a must. Always get it fast so it can stack.

Sorcerer's Shoes or Ionian Boots of Lucidity. Take Ionian if enemies don´t got so much magic resist, and a void staff will do the most anyway;)
Sorcerer´s is always great to melt enemies magic resist. If they fx. got Shen Galio

Rabadon's deathcape Is just super great to kill those enemies on 1 sec;)

Athene's Unholy Grail is nice with its CDR and Mana regen. It also got a good amount of AP;)
When against AD we don´t need that magic resist so there Morello's Evil Tome is fine and a bit cheaper:)

Void Staff is great to melt through enemies magic resist:)

Banshee's Veil got give you some mana and health which is great, and the passive is just awesome against that Mega Inferno Bomb
Against more ad take Zhonya's Hourglass for the armor and also a great passive (if u remember to use it);)

Flash is just great

Ignite For the extra damage, and its efect is also great;)

Remember to be in teamfight insted of farming. When there is a teamfight take out there ad or ap carry, or support if the others isn´t in your range, and then run out and wait for cooldowns before you are going in again;)