LoL, Fiora is at her weakest early game with regards to her passive. It does very little. Early is when you want to punish her, before she has a chance to get the levels and items that make her passive scale well. If you are playing a bully into Fiora, don't be afraid, and don't play meekly. If she tries to proc her passive on you, punish her with your own combo and you will pretty much always out trade as long as you don't do something stupid like blow your combo into her riposte.
Once she gets an item or two though, yeah her passive does start to hurt pretty badly and becomes pretty bullshit imo. I'm not sure why Riot thought it NEEDED to be %maxhp true damage. %maxhp Physical, or just flat true damage would have been much more fair. But whatever, there are ways to beat her if you outplay her. I just don't like her in solo queue because she forces me into a very small subset of picks to deal with her. In a group queue setting its not so bad because you can just bank on a team fight strategy, but in solo you can't really rely on your team mates being on the same page as you the whole game. They are gonna do their own thing so you gotta plan to be able to handle what needs to be handled.