I've been playing nothing but Fiora for the past like, 20 games, and if you wanna check out my thread for more of my opinions on her, you can here. It's a long read, but yeah, there's that. A few little tricks i picked up on though:
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her kit is wonderful for escaping. Her Q resets if you hit an enemy, so in a scenario where you're standing almost on top of your enemy, if you use Q JUST far enough away that it still hits your opponent while putting you as far away as possible, you get the refund for another dash in 3 seconds. If you've got Phage, it gets even better. Her W can also be used. once you're sure you've got enough distance, throw it out behind you for a long range slow. Combo W (assuming you get the stun) into Q when you get ganked for a clean escape. Her E has a slow on the first hit, enough said there, and her Ult gives you a speed boost when in it's radius. Place it on a sticky target that's chasing you to outrun them.
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is a MUST buy item. It synergizes well with her passive and her ult, along with any lifesteal you plan on buying, usually in the form of
. It also adds some cooldown reduction, which Fiora always appreciates.
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Use the slow portion of your E to reposition for a vital strike on the second hit, which crits. I normally take this route for extended trades: Q in for a vital strike, auto attack, E to cancel the animation for another auto attack + slow, wait for the next vital to appear while using Riposte as needed, then crit the vital and retreat.
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Riposte can hit vitals. Use it if a target is fleeing, the vital is facing you and your Q is on cooldown (for some reason).
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In teamfights, place your ult on a target that you know FOR CERTAIN will die. Use your Q to hit a vital, then retreat as nessecary. Eventually, that target will die and your team gets free heals.
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Fiora is - as she always has been - very item dependent. While she does deal true damage, her base damages aren't very high. For the laning phase, stick to short trades, skirting in and out of danger. Remember that the healing from your passive can negate a little minion damage, so don't be afraid to go for those side vitals. Once you rack up some gold and cash in, however, dont be afraid to dive that enemy carry. Choose the right moment, and you'll send them scurrying for the Soraka. Choose the wrong moment, however, and you'll get dogpiled.
Hope this helps. Happy dueling!
EDIT: As an afterthought, it might be a good idea to set Q to quick cast with range indictators so you know exactly what's in range and what isn't.