This is how you win against Darius as Fiora, she is a counter to him if you know what you're doing, first off I'll explain how Darius works so you can work around his abilities.
Darius's Q is his main damage source (besides his ult). He swings his axe in a big circle, the outter part does the most damage, it also heals him and applies a blood stack.
The inner circle does 65% reduced damage, doesn't heal him and doesn't apply a blood stack.
This is your best way of countering him, especially as Fiora who has a dash.
You want to try dash towards him inside his Q, this way you can make a solid trade and just stick onto him, hitting the vitals. You just want to keep chipping his health away.
DON'T carry on fighting him after hitting a few vitals, the longer the fight lasts the more likely he'll kill you.
ALWAYS save your parry for his ultimate if he has 5 stacks, his Ultimate has a animation before it actually hits, wait patiently for it then use your parry.
This is your best bet on killing darius and winning the fight.
Hope this helps!