Delay your build and get a tonne of pots for sustain. Farm from a safe distance and wait for the late game. If you chose a melee-mid, you're in trouble. Other than that, just farm up.
If the LeBlanc is smart, she'll start roaming to get kills, and you need to rely on your team mates to ward up or else she'll snowball off them.
I generally take Karthus. Sure, LeBlanc lanes strongly against Karthus and she wins all trades hands down, but if I can avoid dying to her until level 12+ (in team fights) and take it to late game, the 700 AP Karthus with 3k HP and 3k Mana will wreck the enemy team much harder than a LeBlanc with the same stats.