I main leblanc http://imageshack.us/photo/my-images/854/lolsb.png/ (im not prefect, but better then 80% of other leblancs) and know how to counter her. Here are 4 main ways
Mordekaiser counters LB hard. Passive ignores burst, he farms incredibly fast while LB cant farm, is tanky (usually) and spell vamp (usually rushed) gives him great sustain. If you are LB vs morde you are probably going to lose regardless of skill level. LB can burst you, but if you have spell vamp youll just heal fast with your amazing farm and zone her from farming.
Galio counters LB. Galio gets lots of MR, making her burst useless. Galio is also an incredible farmer with his abilities, while if LB uses her W to farm, she cant get away/gank for 10-20 seconds (dependant on game time) meaning a flash/galio ult will rape her, and she would be an easy gank. Galio should beat LB regardless of skill level differences. If Im LB and get countered by a galio, I just try to keep my tower, not feed, and hope my team can carry me.
Soraka counters LB incredibly hard. Her sustain is immediate, her silence ****s up LBs burst, and even if LB does land a successful combo, soraka should heal enough to get away if shes not fed. Soraka can outfarm, outpush, and sustain while LB can not. And sorakas MR passive makes LBs burst suck even more.
Sivr counters LB. (spell shield ****s her combos/burst, and throwing a Q when her clone pops is a guaranteed hit on her. Spell shielding before a leash can prevent stun, and silence does very little to sivr)
Hope it helps on your guide.