I know she seems scary, but you gotta remember that she is beatable. A lot of her damage is locked behind her sigil, so if she throws a Q at you to be peachy, back off to avoid the W to pop it.
Keep behind minions to deny the E, and whatever you do, do not feed her.
She falls off late, but she will pull tricks and make you disappear rather easily if she gets fed.
You win against Leblanc by denying her kills. That is it. Play as defensively as you need to, get your farm, and just chill. The Leblanc will be bored out of her mind and will try to roam. Do not let her do this. Waveclear and shove her into tower so she has to use her spells to avoid missing farm. Keep your distance and your eyes peeled for ganks and you should be golden.