leblanc requires a specific rune set to even be remotely effective late game and in higher tiers of league because her kit is about misdirection. just like all of the other misdirection champions once the secret to their "magic act" is revealed they become very poor champions.
let me explain further.
her passive activates whether you want it to or not and then goes on cooldown. it also spawns running away which looks really dumb if you're not running.
her R has the downside of it not being a separate spell but also doesnt deal damage = the spell it replicates but instead = to a static number
with the focus on team fights and clarity being embraced by the community leblanc falls off substantially as a single target disruption assassin/mage.
her kit was designed with silence in mind and then once silence was removed from the game she lost a lot of her power.
yes you have to do combos to make up for that lost damage output. if you dont your opponent is going to either get away or kill you.
her Q specifically requires you to use another spell just to deal your Q's full damage output. if you combo 2 Q's together by using your R you lose out on some damage if you dont have a spell up to activate the mark. make sure to combo your Q in and out of your other spells
her W is mostly for repositioning but deals the most guaranteed damage out of all of her spells in both basic abilities and mimicked abilities however it doesnt deal its damage twice like E and Q so you're going to miss out on damage if you use R to double up on it when your target is immobile. you're better off saving it for an execute on fleeing targets or as an escape.
her E deals double damage if it stays connected and prevents the target from moving which is pointless to use if you cant keep up with your target. if you double up on your E too early you waste the first E's root.
a simple combo would be
Q E W - wait for cooldowns - E Q R W. it starts off marking your target with Q then you apply your slow with E which grants you the additional damage of Q and you use W to stop your opponent from getting out of range so you can root them. then you apply your slow again double up on your mark and execute with your W. if you're in danger you can reposition with your W and that will put it on cooldown faster.
there are also escape combos and kiting combos but those are a bit more reliant on movement speed and late game stats. if you cant win the game by mid game then you're going to be on the run in late game.