This guide is made mainly for 3v3. It can also be used for 5v5, but i dont ever play 5v5 with leblanc. There are many better champs for 5v5, but i always use this build for 3v3. At level 6, ur supposed to be able to burst someone till death with one combo. I also recommend smart cast, else you will never land a combo fast enough. I made this guide since i play LeBlanc alot, and almost always the enemy says i am OP as soon as I reach lvl 6. I am sorry if my English ain't perfect, i hope you understand I'm dutch and thats why my english ain't perfect. Please don't rage in comment, just leave some tips for improve or explain why you upvoted/downvoted.
For masteries i go full offense. I take the mastery upgrade at the offense tree, but i dont master flash, even though the 15 sec cooldown could be usefull. You can make this choice yourself. I take all the things that upgrade my ability power or magic damage and i make sure that i master demolitionist, with that i will be sure that i take down towers faster. Also take the increase damage thing, since its pretty usefull.
For items, I grab a doran's ring at the begin, since LeBlanc will be way too squishy without, and u gain alot of things for low cash. I dont grab 2, because that would ruin the rest of the build, and most times u can burst someone down at once at level 6 so you dont need two. Second i grab mejai's soulstealer. With that item you will start great, and every time you kill someone you will get stronger without going back. Then i take sorcerer shoes, because without the move speed you will die soon, even though juking is a good possibility on LeBlanc. After boots i get the Tear of Goddes. After that, I am much less mana thirsty and increase my mana all the time. Rabadon's deathcap after, for alot of Ability Power. Then i build my tear of goddess into archangel's staff, for the ability power. I purchase tear of goddess before the rabadon's, in order to let it charge enough, instead of missing alot of kills due to being short on mana. The next item is abyssal scepter, because the enemy almost always has at least one mage, and the magic resist is awesome. It also reduces enemy magic resist and gives ability power, which makes you even stronger. Most times, the game already ended before archangels staff, But if you get to this point, sell dorans ring and buy Rylai's crystal scepter. This will give you a slow to ensure you will finish your combo or to make you escape easier, and give ability power and health. Really good. When you are all finished with that, purchase elixirs: brilliance first, fortitude after, then oracle and last agility.
On leblanc, i take Q at level 1. I make sure to have taken all skills at level 3, so i have my full combo already. I max Q first, because it deals most damage IF you trigger the mark. I max W after Q because it deals most damage after Q and is AoE. I take R whenever i can, like most ultimates.
For summoner spells I take flash and ignite.
Reasons for this summoner spells and others:
Flash: This is an awesome spell for almost every champ. Great for escaping, chasing, finishing off or getting out quickly.
Ignite: This is very usefull if u are a person who always does like 50 damage too low. Just put ignite on him when he runs or use it to cancel his heal.
Ghost: Same like flash, depends on your style.
Clarity: This could be usefull, just if you take this, Don't take archangel's staff but change it into void staff or something.
Dont take this:
Exhaust: Dont ever take this on a burst caster, since it will waste your summoner spell slot.
Heal: This is only for when u are really low level, if you are level 12+ then you should be ity:able to take better spells.
Pros / Cons
Alot of burst damage
Not too hard
LeBlanc is also great at ganking, and will excell in quickly getting down weaker targets, flashing out and then coming back with a great burst combo. In 3v3, do it like this:
When you are at top lane, push untill your enemy is close to his turret. Then walk back, like you are recalling. walk to the bush at bottom lane, and wait for the enemies to push. Walk in, burst down one of them, and walk back to the red buff. Walk on to the next bush, on their side, and attack again when u have your combo ready. When you got these two kills, your team should be able to be carried by you. Also, place a ward at dragon. When they almost have dragon, use W (distortion) to get in, land Q(sigil of silence) and E (etherial chains) to KS it, then use W again to go back. then you earned another 280 cash for everyone!