fiora vs panth is a pretty bad match-up but there are very few things you can do.
Mostly you have to just try and keep your cs up since you are a better teamfighter, and will out scale him latter.
Also in lane pay attention to his mana if he uses Q to poke too much he will go oom and you can go agro the second he does.
Third ask your jungle for help since panth is fairly immobile and easy to gank, if your jungler can get you ahead you'll easily take the lane over.
Buy plenty of health pots everytime you go backit actually helps quite a bit if your constantly potting up.
There is also a kind of cheese I like to do with fiora it sometimes works if the person is unprepared, if you skill E first you can push quite hard and hit lvl 2 first and try for an all in, just be careful to pay attention to minion agro, and how close panth is to lvl 2 (u don't want him hitting two in the middle of your all in).
hope this helps