Solo champions - wait for your chance to strike, either from the jungle or the enemy champion has used most of there powers (recommended against AP champions). You should survive most melee/AD encounters with life steal.
Team fights - wait for a tanky character to initiate fights and then jump in for the kill.
Try and play Fiora as an assassin, more than a fighter.
Go all in with that offense, or split between offense and utility; don't go for tankiness. Fiora is all about poking in and out on her target until she can go in for a kill. She's a duelist, not a bruiser. --- This isn't 1950. Girls develop way later than they used to because human lifespans have increased - Lavos_Fanboy