LeB works best as solo mid or solo top vs another solo, as she has some good harassment against a single target, but is too squishy to be able to stand up to a strong opposing lane. She's also fairly weak at pushing, needing to use her escape/chasing tool if she wants to farm, making her vulnerable.
Early harassment is Mark->Distortion. You want the Mark first so that the Distortion damage can activate the silence+damage aftereffect of the Mark. You can additionally tap Distortion again to get back out of range, or sit and auto-attack if you arn't in much danger of retaliation - given that they're silenced, they'll probably be running for a bit anyway.
Ganks at 6 should be roughly Mark->-Mimic. The Mimic'd Mark will activate the first silence and extra damage and results in some scary burst. Use either Chain or Distortion to activate the Mimic damage - Chain if you're confident you can hit the right target with the skillshot, Distortion otherwise (Chain after Distortion if you need the extra damage). That should result in most targets falling over, continue to chase with the slow/root from Chain and auto-attack along the way, finishing with a final Mark. Just watch your 6, you're vulnerable while waiting on cooldowns.
During Teamfights, you need to do 1 of 2 things, dependant on enemy team comp and the situation. If the enemy team has a squishy, high damage target, wait for the fight to start, walk over and blow their face off. Watch for and hard control heading for you, and Distortion out if you have to - you don't need to kill the guy (though it's prefered), just mitigate his presence during the fight.
If the enemy team has multiple high-value targets, determine which one of them screws over your team's damage dealers the worst and... deal with them. Remember that you can often silence a victim before they know what's happening - watching Pantheon fumble around, trying to Aegis stun you while silenced is pretty funny. The key idea here is you have the firepower to shut down one target. Pick wisely..
Once you've blown your load, hang back a little and wait for cooldowns. Don't be afraid to poke stuff with your auto-attack when it's safe - it's not much, but it's something. Once Mark comes back up, wait for either Distortion or Chain and silence/snare/root/screw another target. You won't blow them up like the first guy, but it's still a good chunk of burst, and they might already be low from the fighting. If you have a choice of targets, the one with strong CC or other, save-my-team or screw-a-victim abilities should catch the silence combo.
Even if the enemy team has no easily-explodable squishy for you to pick on, you're still trying to look for the dude that screws your team's damage the worst. Try to silence them asap to prevent them from doing their thing. Silencing Xin can be funny - he expects to slide in and unleash, but instead he's slowly walking into your team at half-health. Much less dangerous, no?
LeB's general role is disruption. Either by blowing up a key player, or by using her silence/slow/root to screw with a key player's plan.
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