Hi everybody :)
I started to play 
and every single game I play with her I get pretty fed during the laning phase then i get literrally destroyed because I don't know what to do in the late game and teamfights. Being a 
main i'm used to splitpush, but I don't feel i can do the same with because she is AP. So, any advice to give me? :)
(Sorry if my English is kinda bad, i'm french canadian)
how to assassin
Catch squishies out of position, shit on laner with insane poke combos, pick up blue buff, shit on lane some more, catch more squishies. Pretty much every mid there is.
Refer to SK vs FNC in the EU LCS earlier today, where SK Fox's Leblanc ended 9/1/3 or something ridiculous - his only death coming around the 27 minute mark from a 1v5 dive where he still dished out absurd amounts of damage to 3 targets on Fnatic before being shut down by their Rumble. He spent all game shitting on Ezreal mid and roaming to bot, I don't recall him going top to attempt to go up against Rumble with Gnar at all but rather looked to pick up squishies in bot lane and mid.
teramfights aren't your thing as LB.
win the teamfight before it happens i.e. pick off adc farming before he gets with team for mid push.
You can team fight with LB very similarly to how you would with Zed. Wait until you see some of the CC spells from the enemy go off, the dive in and try to blow up someone on the back line, either you kill them or force them to gtfo, and you can just w right back to safety. You're ult is even super low CD so you can use it early in a team fight and then again to chase someone down and finish them off or to surprise someone chasing you with quick and embarrassing death.
LeBlanc is one of the worst splitpushers in the game...so don't try. You're a pick assassin...so you never want to dive in on more than a 1 on 1. LeBlanc's roam is amazing....so if you win your lane...don't push a tower...let your minions do it for you and take the time to roam top or bot and look for pics. Your disengage is amazing...so don't just waste your W/R combo unless you're going for that guaranteed kill.
