Duelist's regeneration effect stacks up against champions, so don't be afraid to engage in small skirmishes.
Because Crowd Control is a vulnerability of Fiora, consider taking the Cleanse summoner spell.
Lunge is a mechanically simple skill, but it has a lot of nuances to its usage.
Lunge is often underestimated for its damage, especially at early levels of the game. A quick double click on the same champion works as a decent nuke.
Remember that while most of Fiora's damage comes from her E, Lunge is an ability, so it can pierce through statuses like blind and Counter Strike to deal damage.
To maximize damage output, apply as many autoattacks as you can before using the second Lunge.
Activate Burst of Speed before using Lunge for best results; the extra movement speed will gain a stack from Lunge, allowing you to continue attacking the target, and finally use the second Lunge to further damage the enemy.
Make good use of Lunge's second cast to stick to your target; if the enemy has their own mobility or escaping skills, try to force them to use it prior to your second Lunge cast.
If you're chasing an enemy champion that's too far use Lunge on an enemy minion and use its second dash to catch up.
You can use Lunge to dash through walls, though you have to be able to see your target. Placing a ward near jungle mobs can allow for an easy escape route.
A good harassing strategy is to use Lunge's first cast to go in and immediately using Riposte (you can even use it mid-air); an opponent's most normal reaction is to attack you right after you lunge, triggering Riposte damage. Land a basic attack or two, then use your Lunge second cast into an enemy minion that is further away to escape. This is only effective against melee champions, as ranged can still counter after you retire.
Don't use Burst of Speed in this way as you only have a short amount of time to land, at best, one or two attacks and back off due to Lunge second-cast short timer. This is unless you intend on killing your opponent, then use Burst of Speed early and save the second lunge cast to continue the chase.
Try to use Riposte when the enemy is already in half-swing or when their ranged basic attack is in midflight.
Against melee champions Riposte is much harder to successfully parry an attack with, due to the near-instant attack time. Unless your opponent is predictable, it may not be worth it to bait attacks hoping to apply the parry damage.
Champion skill order is not set in stone, and varying it depending on the game's situation can make you much more effective.
Skilling Lunge makes it gain a decent amount of damage, but more importantly greatly reduces its cooldown. This can make all the difference in chasing opponents, especially those with mobility skills that could otherwise evade and kite Fiora. Max this first by default, as it’s the most reliable way to increase Fiora's skirmish damage against most lane opponents.
Riposte improves Champion offense while providing a situational defensive move. Parrying attacks constantly, especially enhanced attack abilities, can save your life. The passive AD bonus is also quite sizable and improves all your other abilities' effectiveness to a small degree. Max this first when fighting champs with good poke and sustain, such as Gangplank and Nidalee.
Burst of Speed becomes an incredibly potent steroid when maxed out, and most champions will be unable to keep up with Champion sustained damage while it is active. However, it is easily countered by saving crowd control abilities until after it is activated. Max this first when jungling, and when your lane opponent lacks CC; you'll usually want to max this second otherwise.
Burst of Speed gives a huge attack speed boost so it can be used to effectively push turrets.
Blade Waltz not only deals damage, but also prevents targeting from enemy abilities.
Blade Waltz makes you untargetable for the duration of the ability and also removes turret aggro from Fiora, so it can be used to finish off low health enemies hiding under their turret.
Blade Waltz basically counts as 5 separate basic attacks. Hence, it will proc all on-hit effects and be affected by any other autoattack modifiers (except for attack speed buffs/debuffs and crits), including the summoner spell Exhaust. However, all proc on-hit effects will stack after the first attack, granting only 4 procs (the first hit starts at 0, the second at 1, and so on, etc.)
If you are caught at low health and have strong life steal, use Blade Waltz to deal massive damage and restore a large portion of health from it. Although you are protected from targeting for the duration, be aware that Damage over Time (cast before Blade Waltz) will still damage Fiora.
Ravenous Hydra shines on Fiora, giving her a significant boost to damage, out-of-combat sustain, in-combat sustain, and waveclear, as well as allowing Blade Waltz to deal splash damage to nearby enemies. It's also easier to build than The Bloodthirster, as its individual components each cost less than 900g to purchase; build early if you plan on using it, as this will be your core AD stat item. Don't forget to use the active ability!
If building Fiora strictly as an assassin (rather than a carry), her role becomes picking off low health and/or high priority targets, making Tiamat or Ravenous Hydra debatable choices on her even with Idol of Durand on your team since they cost a fair bit of money for items that don't tremendously augment your single-target damage, especially when Fiora's primary targets are often back-lines characters who won't be standing near enough to anything for the splash damage to actually hit anything.
With Burst of Speed, Fiora can down towers very fast. Ravenous Hydra can be useful when used to speed up Fiora's wave clearing. This can turn Fiora into a very effective split pusher; if you're at the point where you can destroy any single enemy champion 1v1 in a duel regardless of tower proximity, this can force the opposing team to commit numbers to counter you, allowing your team to secure other map objectives.
The Black Cleaver will reduce armor on every hit of her ultimate Blade Waltz and therefore increasing the damage dealt. In teamfights where your Blade Waltz can hit multiple targets, the armor reduction will be very helpful. Combine with Tiamat/Ravenous Hydra to shred armor on any champions struck by the splash damage, potentially granting maximum Cleaver stacks on every enemy champion in a teamfight and boosting primary target damage on each strike. In a 1v1 case, the armor reduction will greatly increase your already impressive damage from Blade Waltz. The additional health is welcome too!
Last Whisper as a third or even second item can massively increase Fiora's damage output, particularly from her ultimate Blade Waltz (which enjoys high base damage). Even early on when combined with armor penetration runes/masteries and The Brutalizer, the resulting 22 flat penetration and 35-45 % penetration is enough to hit most unarmored targets for true damage. With a Ravenous Hydra or The Bloodthirster, you can often "100 to zero" enemy carries with just five strikes of your ult, without even factoring in the rest of your abilities.
Building life steal is very important for Fiora as it allows her to regenerate large quantities of health while she is untargetable during Blade Waltz.
Building more than one lifesteal item makes Fiora extremely difficult to 1x1 and allows her to go from nearly zero health to full health if she uses her ultimate, as well as allowing Fiora to quickly renegerate by attacking minions or monsters. This is viable because Fiora has an huge attack speed steroid in Burst of Speed, allowing her to win almost any 1x1 duel, by quickly lifestealing all damage done to the enemy.
However, it may be more effective to just build one of those items, and build critical chance items, because they effectively double Fiora's lifesteal, but allow her to do much more damage than by building the above items. Be wary that this relies on Fiora actually hitting for a critical strike, which means that having more than one critical item is recommended, which the best options being Youmuu's Ghostblade and Infinity Edge.
Statikk Shiv normally gains 10 charges per attack but the 5 attacks of Blade Waltz also count as movement by various amounts. It is fairly common for Blade Waltz to proc the Shiv with the first strike, yet the Shiv will have 75-100 charges by the end of it.
If one is running Armor Penetration runes, it's a good idea to get both The Black Cleaver and Youmuu's Ghostblade on her, since she can receive the full stacks for the armor shred easily.
Bilgewater Cutlass can fill a powerful niche in Fiora's kit, granting her good life steal, decent damage, and a slowing active. Upgrading into Blade of the Ruined King further improves the life steal and active while also granting just the right amount of additional base attack speed Fiora needs. Consider buying this item early against teams with high-health champs and keep away comps, as it mitigates many of Fiora's weaknesses and allows you to focus the rest of your build on raw damage output.
Wit's End is also useful as it will proc on all hits of her ultimate, Blade Waltz, without the 75% reduction for hitting the same target. This effectively adds 210 magic damage to Blade Waltz. Along with the magic resistance and AS that it provides, the item is very useful on Fiora, especially if the opponent team is AP heavy or if an AP heavy champ constantly focuses you.
When jungling Fiora, Spirit of the Elder Lizard makes for a good default pick; particularly if you plan on ganking often in the early and mid game. Wriggle's Lantern is also a good choice if you plan on focusing on jungle farm, though it necessitates an AD heavy overall item build; each Blade Waltz strike will proc bonus damage.
Due to the nature of Fiora's kit, many types of boots are useful on her.
Berserker's Greaves are overall the most common choice, as they provide more attack speed which is overall useful to Fiora, especially when combined with Burst of Speed, as Fiora is very auto-attack reliant.
Ionian Boots of Lucidity is an equally good choice, especially if you are building Fiora more as an AD caster than a melee ADC. The cooldown reduction allows her to use Lunge and Blade Waltz much more often. Remember that out of all melee ADCs, Fiora is the least attack speed reliant of all them.
Ninja Tabi and Mercury's Treads are effective choices if Fiora is losing the lane, the latter is also effective if Fiora is being focused by all enemy crowd controls, which is usually very likely.
Fiora is adept at dealing a lot of physical damage quickly, making armor a clear method of protecting yourself.
In the top lane, Ninja Tabi is a highly effective way to become deceptively hard to kill to Fiora.
Buying Thornmail is a very good option as Fiora relies heavily on basic attacks to cause damage and it would benefit almost every member of the opposing team and increase their survival chance.
If you are a squishy champion, avoid trying to 1x1 Fiora unless you can kite or quickly burst her, as she excels at winning duels due to her insane damage output, attack speed steroid, and an ultimate that does massive damage in addition to making Fiora untargetable.
Instead, try to always be near your tank/bruiser/support. If they have hard crowd control, this will make Fiora very easy to kill as she is very squishy.
A crowd control effect applied on Fiora when she activates Burst of Speed will severely lower her damage output. In addition, if she is unable to autoattack or use Lunge during the 3 second duration, she will not gain bonus movement speed.
Fiora is considered a late game AD hyper carry; as such, she must be shut down at every given moment, as she snowballs deadly and quickly
If Fiora runs bottom lane, focusing her early and often with crowd control and heavy damage is a great way to shut down her mid game, due to Fiora easily (Much more so than other champs) falling behind from not getting early kills or assists.
If she uses Blade Waltz recklessly, you can run towards a friendly turret and CC her upon reappearance.