I was playing ARAM and was fighting against a team that had Fiora.
She was not building tanky and we were only level 12.
A single game is a terrible place to draw balance QQ from, let alone an ARAM game.
ARAM being a useful tool for balance only really extends to comparing ARAM win rates to Rift win-rates to see how the different maps highlight different champion capabilities (IE sige and team-fight potential). That's pretty much it.
Otherwise the team-compositions are randomized rather than carefully selected, and the skill level of each player on each champion is randomized rather than set to a high level.
It's not hard to pick out an Udyr or Warwick main when they actually hard-carry their team in ARAM despite being at a clear dissadvantage to the likes of Lux and Ziggs, practice on a given champion makes a massive difference.
Also - Fiora is a glass cannon. She never builds tanky. She has more hyper-scaling damage potential than Vayne, Kog, Jinx, or Trist and pretty much no damage output if she tries building tanky (where tanky DPS champions tend to have hybridized damage that acts as a free Last Whisper or Void Staff - usually along with %armor pen, true damage, or %HP steroids).
How is it possible that Fiora can tower dive TWICE and where the second tower dive is 2 turrets, with 3 champions using skills on her, without her using her ult and STILL get 1 kill when we were all at full health AND she had no allies around her?
Most assassins can kamikaze with a reasonable chance at success.
Pretty much every melee has low enough casting times they can QWER before the cast and projectile speed of a lot of CC effects can stop them.
You're also just saying dove into a team and killed a guy - if you're sitting at 40% HP as a squish and get into a bad position, it's not exactly difficult to trade 1 for 1 in such a scinario. Champions like Lux, Ziggs, Cait, and Xerath can just pew-pew you with abilities and ramp up to their legendary streak.
Don't even try to deny the fact that any other champion would have died because it is downright broken and completely ruins balance in the game.
Uh, Lux, Ziggs, Xerath, and Cait wouldn't even take any damage in return do to their reach and capacity to dive without issue.
Also - a wide range of fighters, assassins, and fighter/assassins have some pretty impressive mobility skills, and many have some pretty solid defenses as well.
Nocturne can ult from halfway accross the map then take 0 damage and 0 CC duration from a Tibbers stun-to-the-face, Yasou could skip accross an entire team and shut down a lot of projectile-based skills with a solid wall to reach a target, Poppy can just turn invulnerable and immune to CC, Talon can easily back out after a kill due to stealthing after his combo, Zed just appears shoops back to where he started his combo after he's done.
Either fix her or at least give the other champions some edge to make it more balanced, cause right now, any match with Fiora wins you the game.
Fiora's win rate in Abyss is only ~47%.
Her win rate in Rift is only ~53%.