Mirror Image's clone acts like a pet. It will take commands similarly to Summon: TibbersSummon: TibbersAnnie. It is wise to manually control it for the means of confusing the opponent, as a clone heading in a nonsensical direction after being summoned clearly betrays the real LeBlanc to the enemy.
Mirror Image's clone is a champion type unit and therefore can also be hit by abilities only able to target champions such as Ace in the HoleCaitlyn. Take advantage of this by trying to encourage your enemies to use their powerful champion targeting ultimates on your clone by putting yourself out of sight and controlling your clone like she were the original.
Keep in mind that your clone's auto-attacks deal no damage which can give her away if an enemy notices this.
Distortion provides LeBlanc with a deceptively large radius of threat.
Distortion is an effective method of harassment, as re-casting the spell will cause you to flash back to your casting position, giving an opportunity to deal damage with minimal risk of taking significant damage.
If you are being chased, throw Ethereal Chains on the pursuer for the slow and potential snare.
When you end up in a team fight, you should consider utility of your spells over damage, and consider using Ethereal Chains as your Mimic spell for a double slow and longer snare.
A quick way to cast any of LeBlanc's spells is to smart-cast them, either by remapping smart-cast spells to Q / W / E / R or holding shift and then hitting Q / W / E / R. When combined with the combo above you could potentially drop an enemy carry in one burst before they have time to react.
Activating Sigil of Silence's silence mark with Ethereal Chains synergizes perfectly with its delayed snare, preventing the target from using Flash or other escape mechanisms for more than 3 seconds. This should allow yourself or your team to secure kills.
Chaining Sigil of Silence with Mimic can provide good harassment and long silence periods early-mid game and can shut down troublesome casters later on.
By selecting a target to cast Sigil of Silence on while out of range, you can use Distortion to close the gap, and then use it again to return to safety. Doing this can extend the range at which you can safely harass and ward enemy champions from gold or experience.
While it may seem unintuitive, Athene's Unholy Grail is actually one of the best core items for LeBlanc. It affords her some cheap magic resist to increase her durability in lane, virtually eliminates her mana problems, and most importantly it provides her a large bonus to cooldown reduction, which, because of her high base damage, drastically increases her overall damage.
Mejai's Soulstealer can be especially powerful, particularly if mid-laning. Be warned that due to her already snowbally nature, getting killed while using this item may cause her to fall even earlier in the game than necessary.
It is worth noting that getting this item will likely make the enemy team redouble their efforts to shut LeBlanc down. Given that she is already a priority target early particularly if she does well, getting this will make you even more of an early-game target while delaying your build for an item that may not even reach gold efficiency if you are focused down.
Deathfire Grasp can be very powerful for LeBlanc as it covers AP and an additional spell in her combo with its Active, which will make her more of a threat in the later stages of a game.
Haunting Guise can be a good early game choice for LeBlanc, as it increases durability and damage, while also negating early game Magic Resistance. It can also replace Sorcerer's Shoes in many builds, allowing the use of other boots without sacrificing Magic Penetration.
Two Doran's Rings during early game provides cheap and highly effective stats.
Investing in an early Negatron Cloak for magic resistance will significantly reduce her burst damage. Later in the game, you can build it into a Banshee's Veil and disrupt her combo. Otherwise, if you are playing a squishy mage or AD carry, she will be able to burst you down even late game due to good scaling. Do NOT neglect MR.
LeBlanc relies heavily on spell combos, especially with her Sigil of Silence and its mimic. Try to interrupt her spell combo in the middle of Sigil of Silence, as this will often force her back to wait for cooldown.
This makes Banshee's Veil particularly effective against her as she has to land one of her non-targeted abilities before going full combo.
Be sure not to use a champion who relies on skillshots or delayed AoE damage to fight her, her quadruple moves (with both Distortions) can make her untouchable by champions such as Lux, Xerath, Diana, Ziggs or Zyra.
Try to scare LeBlanc a bit to force her to use her dashes prior to a gank , as the long cooldown will make her an easy target.
Mirror Image's clone does not carry buffs- use this to identify the real LeBlanc.
Always wait for her passive to trigger before targeting her with an ultimate like Demacian JusticeGaren because otherwise she will most likely get away with Distortion.
LeBlanc is extremely squishy so she is very vulnerable to hard cc and focus. Taking her out of the fight early prevents her from being able to burst down a carry.
Mirror Image's stealth does not break the auto attacking AI's lock on her; allowing the autoattacking AI to continue without interruption can finish her off.
LeBlanc is notorious for being able to burst down AP mid laners. However, because she specializes on bursting one target, having your jungler gank during her dives will cripple her aggression significantly and cause her to fall off prematurely.
Warding brushes will make it harder for LeBlanc to juke using Distortion.
LeBlanc has poor farming capabilities; if you're unable to make trades safely, then simply clear minion waves quickly to force her back.