You could focus on getting squishies first but make sure you do not dive too deep into the enemy's backline most especially if they have a hard cc comp—your riposte wouldn't always be enough to save you from dying.

In my case as a Fiora main I like to just play safe and wait for the enemy team to engage. The reason behind this is that I could easily shred anyone (including tanks) with my damage output. Another good thing to put in mind would be most tanks have little to no disengage.

When it comes to prioritizing squishies first, I have nothing against that but all I'm trying to tell here is that Fiora couldn't really engage/disengage easily after going to the enemy team's backline especially when they have hard cc or a good peeling supp—e.g.- Janna, Soraka.

If you want to go for squishies first, here are some things you need to put in mind.

1) Make sure you could engage and disengage easily

 * In this first point it's obviously the first thing you need to put in mind. Make sure you could reach the target easily,  assassinate them and get out of danger as soon as possible. 

2) Make sure you could kill your target/s before the enemy team could even possibly peel for them.

 * In this second point you need to put in mind the enemy team's comp. If they have hard cc or an incredibly effective peel champ, you need to stay away for you could die easily even with Tanky Items. 

3) Make sure your target couldn't escape easily.

 * This point should always be put in mind. For example you're trying to go against an Ezreal and Janna backline, staying away from them until you destroy their frontline would most probably be the best thing to do as Fiora couldn't easily engage these two champs UNLESS they're out of position.

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Remember: Positioning is one of the most crucial things during teamfights. When you make one wrong decision in diving someone on the enemy team's backline, as a carry it would probably cost you the teamfight or the gsme itself.