How to hit vitals in clumped up teamfights as Fiora?

Hi there, I’m a little confused as to how to teamfight with Fiora.

When the enemy team is clumped together, I find it impossible to single out an enemy and hit their vitals, because unless you are a mechanical god, how are you supposed to avoid hitting other enemies?

Not only that, but it can be impossible to see exactly where the vitals are, unless the enemies are at least a little spread out.

Should you just skirt around and hit any vitals you can in these situations, and not focus a single champ? And maybe drop your ult on the enemy who is about to die?