You have to play her as exactly what she is. An assassin. Go for the carries. And if you get them, awesome. If you don't, you've probably just drawn massive focus away from other members of your team like, say, the late game top laners that work very well with LeBlanc like Jax. Just go to the back, (Try lurking to the sides of fights so you get an easy shot at the AD/AP), then swoop in, burst someone out of the fight, run, get CD's again, and go back in. Also, work with your support to get good ward coverage in their jungle, because LeBlanc in an amazing ambusher. See their AD going for their red? Set yourself up in the Wraith bush, and jump his ass when he starts walking back. ALWAYS be ready to capitalize on bad positioning, even if it's just the support sweeping for wards. Kill everyone that separates. You're gunna want to abuse fear like people would with old Eve or Twitch. They won't want to split, because they don't know if you're waiting for someone. Oh, and there's nothing wrong with going Flash>Q>R>W>E then dying in a late game teamfight. If you get the AD in one of those "Must Win" team fights, you've just traded kills IMMENSELY in your favor.
TL;DR: She's an assassin. Play her as such. Ambush hoes in the jungle and dive for the AD.