I've been trying LB out, and have had a lot of trouble carrying games on her - my record so far on a low level account is something like 2/11 wins when I have no problem winning games on champs like Alistar/Singed/WW.
Apart from long-ish cooldown single target burst, I just couldn't get much done unless I got a lot of early kills that I snowballed with, since squishies seem to survive QR>W/E combos with a little health (with a sorcs>guise>deathcap super glass cannon build) and I don't do much to bruisers/tanks.

tl;dr how do you win games as leblanc?

Dom is about team work and not Carry... you might want to change your mindset and play for a team (then taking all the kills and then complain LB is weak late game)

The guy asks for help with how to play a champ and you jump all over him like he said something wrong. A carry is a necessary part of a team comp, and wanting to be successful at it doesn't make you selfish.

Sadly I have no LB experience so I can't offer any advice to the OP.

Sorry, it's about the mindset here. So many people thinks they can carry game and fail, seen it numerous times....

Anyway, LB has only single target burst. LB is good counterpick to some champs, but not very useful in Dom. Also if the game drags out, your damage will drop...
If you insist of playing her, pick their squishy, finish it in 1 combo and get out of there... use your snare utility to help your team beat up others.

All I will say is: her build, her build, her build, her build and maximizing your W and R. Contrary to popular belief, her Ultimate will mimic any of her other spells. She works best when shes the only main AP going top. This is for a few reasons:
First: She is significantly gimped when they have over 100 MR.
Second: It forces the opposing team to either stack MR, whereby making the rest of your team more useful since presumably they are mainly AD or deal with always being down a person (due to being dead due to lack of said MR stacking)
Third: At the very least the opposing team will be forced into either a total offensive team fighting or total defensive team fighting. In other words skirmish tactics will fall heavily in your favor (Notice I did not say "poke" tactics).

If you want some ideas check out Big Mclarge Huge or LaRoue(I think I spelled it right) as that is whom I learned potential builds as well as devious ways to maximize her potential.

I play her just fine on dominion. heres my build:
boots prospector ring 2 hp pots
first buy needless or blasting
rabadons
sorc boots (when against mostly squishy team)
Ionian boots (against team tank)
Will of the ancients
from here i build accordingly, ap items sometimes a thornmail

for runes i get
all ap yellows blue and quints.
mag pen reds

she has so much cc that you dont have to build her glass cannon on dominion. her chains is a great snare she has escape mechanism and excellent burst. just stick behind your tank dont get caught and deal that dmg. dont be scared to pick up odyns or thorn. its great extra dmg and survivability. but dont go and get them everytime only when the situation calls for it.

15% spell pen and 450 ap is enough to be a great damager and thats really all you need. void staff is lovely though.

I love LB on Dom, so long as the other team isn't all bruisers. Not that you can't do anything to bruisers but they are for more time consuming to kill, which increases the chances of them getting backup before you can finish them off.
My core build is this:
Prospector+boots+hp/mana pot. I find I make use of both during or after the windmill fight and it allows me to stay out there longer.
Morello's. You need to get 40% cdr. I don't know how anyone can not have 40% cdr on LB and have success with her. And contrary to many other champions, she can really use the mp/5.
Sorc Boots. Always a good dps increase, and you shouldn't need any other shoes. Ionian only gives 15% cdr, which isn't enough to cap with runes and masteries so I find they are kind of a waste.
Death Cap. Self explanatory

From there, it's situational. Most often you will want to build a Void Staff, but if they don't build MR, I like getting a Rylai's for extra CC.

I use AP quints, MPen marks, Armor seals and cdr glyph, with 15/15/0 masteries. This gives me just shy of 18% cdr without items, and thus 38% with just morello's.

Last thing is, you really need to be able to kite to play LB well. Doesn't mean you have to be a good kiter in general (I'm a terrible kiter with anyone but LB and Ryze) but if you can learn to use her chain well, you'll be able to kill many more targets. Also, maximise the use of Q. Getting the proc is important so instead of spamming it as soon as it's off cooldown (unless it'd kill the target regardless), wait for a second spell to be nearly ready. And the second part of chain can also proc the damage, not just the first part.