First off, Fiora isn't just a great fighter, shes a one in a million, a prodigy. She beat her father, a presumably very skilled fighter at very young age(likely about 16), and even if she wouldn't have won, was going to fight the Champion of House Crownguard(likely Garen), so that shows her determination, which is something Jax also has. Skill wise she has trained her entire life to fight, Shes lost track of the people shes killed, which were all warriors or nobles(who are trained in swordplay). In her story fight she kills her opponent in under a second.
Think about it, the characters in league are all falling into one intertwined story that will shape the future of Runeterra, if Fiora wasn't standout amazing, more so than a great fighter, why would they even bother mentioning her?
He probably isn't even Human, and whatever he is he is observed as a "giant"
Jax is likely a human, just a big dude, like Darius or Garen. The pre transformation description of Jax is given from another soldiers point of view. If you were a new recruit in Darius' regiment and you saw him, you would describe him as a giant. He tall, hes incredibly stocky, his bicep is bigger than most torsos, but hes still human. A new soldier, someone unfamiliar with him would describe him as a giant, its the same for Jax.
He BEHEADED AN ASCENDED GOD WARRIOR
When Jax decapitated the ascended, he didn't do it alone in one on one combat, he had his group of elite soldiers with him, they even say "We had to topple half the mountain to put the big bastard down,” Colgrim added. “And even then, only Saijax had a weapon big enough to take its head off.” Now that sounds like the entire unit had to do it, not just Jax.
His abilities imply he has some form of magic, but his abilities weren't created with his modern lore in mind.
They say he wields the "last light of icathia" which is presumably where the magic he gets is from. But even if he has that, Fiora has Demacian steel armor which is, as Riot has made clear, impervious to magic, so that doesn't matter.
Jax has probably travelled across presumably all of Runeterra, finding warriors and observing countless strategies.
Fiora is also incredibly smart, its in the first few lines of her lore, Jax isn't the only one to know multiple strategies.
If her abilities are translated into story, she can easily and quickly identify where her opponent is currently most open and exposed, and her strikes are precise enough to penetrate/avoid any protection they have on. She is nimble and quick, able to easily reposition around her opponent while striking quickly and precisely. While probably most importantly she also has a strong defense, able to avoid/counter any strike be it through parrying it or simply moving out of the way.
Fiora is a challenge for Jax because he hasn't fought anyone like her, he may have fought strong opponents, but not one that was as skilled as her. If she wasn't noteworthy she wouldn't be in the story of Ruenterra.
