Team fight tips
Before engaging in a team fight make sure you have your ultimate because it can change the tide of a whole fight. From the hydra you bought fiora's ultimate will do much more damage.
Also make sure someone tanky jumps in first to avoid getting silenced and being taken out early before you can even ult. After ult target the enemy champ with the least health. If they are all tanky then target the one that ur teamates are.

Always use your ravenous hydra buy click the slot you bought it in because one again it does AOE damage which damages everyone around you and has life steal.

what not to buy on Fiora
I see a lot of players build Frozen Mallet for health. This make you stay alive in team fights more but will decrease your damage out put. Fiora needs full on AD to make a difference in a team fight as she can cut down the anyone who stands in her way.

Also don't go infinity edge as i personally think she does not need the critical strike and should instead buy a black cleaver for damage, cool down reduction, a little bit of health and the unique passive which is very good on her.