she wins level one if she's good because she should walk up, auto the vital and cancel the auto with a q backwards to make the garen chase into minion aggro
as long as she's not stupid and doesn't sit and take q + more autos she should win level one, but after that she eats shit until 6 where she wins unless the garen fakes you out and makes you riposte ult
fiora easily wins the matchup late game, even with decisive strike. unless the garen is 10/1 she should be able to sustain long enough to kill him without going low enough to get executes, meaning she can either riposte the silence or several ticks of your e + she can get ult off before garen is in range to silence
i've played the matchup a lot because for w/e reason people pick garen into her repeatedly in gold 5 and don't learn