if you are an adc you cant touch fiora because she can just q to u and ult u.
Depends on which ADC. Quinn can give her a really hard time.
Generally speaking though, ADCs are horrible duelists and really solid in large groups.
You're utilizing loosing strategies if you're trying to 1v1 someone like Xin'Zhao, Jax, Irelia, Diana, Yasou, Master Yi, etc. with a marksman.
Dealing with Fiora is not different at all from dealing with Jax, outside of Fiora being more like Diet Jax.
The goal of a marksman isn't to duel people or isolate them though. That's the goal of assassins and fighters. Goal of a marksman is to pew-pew things at range. And when you've got 5 members that can each shave off a solid 20% of someone's HP in the 1000 to 0 range journey, the pew-pew is gonna be awesome.
Isolate yourself with an assassin, you get ~20% of their HP before they reach you. Then you're range offers no advantages, and, well, Riot balanced Range as though it's a really important stat that will get demolished if you don't use it.
Anyways - ultimate goal of LoL is to blow up the Nexus. It isn't to 1v1 Street-Fighter tournaments everyone. Some champions win via 1v1 power, others win with good seiging potential, objective control, team-fight dominance, etc.