|With the release of the Nameless Blade in 2.4, Fir suddenly becomes a viable damage unit with many good build possibilities. Rather than scrap all the hard work I made on my build prior to this update, I'll simply modify it to show a way to build her that doesn't focus purely on Attack. IVs are rather flexible here as they don't vary the results that significantly. +Spd/-Res might be the best combination though. While the previous build of Res-refined Slaying Edge+ could be made strong (especially defensively), the Nameless blade is unquestionably stronger/better on its own and allows Fir to do better when initiating combat. Use the Special refine and now it's a Wo Dao and a Killing/Slaying Edge in one. Like before, I'll cover both offensive and defensive takes on this build.
As with most melee units, you want Reposition to get whoever you're saving behind you.
For our special, we can go with Glacies, dropped to 3 turn CD thanks to the Nameless Blade. This is further helped by Flashing Blade 3 building charge every attack so long as she's at least 1 point higher in Spd, which she will be against the vast majority of foes. Due to this combo, Fir can often get her special to trigger on doubles if she's countered (provided the enemy doesn't have Guard). For a more defensive style of play, you'll want Warding Breath as your A Passive which accelerates her special when she's attacked rather than when she attacks. It also strengthens her special by 4. If going this route, you'll get more out of Iceberg as your special since Glacies doesn't activate as often without Flashing Blade's help.
That brings us to Wrath... This is another chance to bring about her Special faster when she's hurt, such as getting attacked on the enemy phase. The real boon here though is the +10 damage on specials, stacking with the +10 you already received from the refine on the Nameless Blade. As Wrath can be difficult to obtain (only available on a single 5-star unit), Vantage can also be a viable option. You won't kill as many enemies due to the damage decrease on specials, but the drop off isn't TOO bad, and you might be able to clean up units that would otherwise finish you off on the enemy phase.
C Passive has a lot of options that should be molded to what you need for her place in your team. If her role is more to support better damage-dealers, a buff would work well here. Otherwise, a Ploy would be a nice option.
Speed +3 is the ideal seal here as it helps activation of Flashing Blade and chances of doubling. If not using Flashing Blade, you may consider Quick Riposte (especially if your boon is already Speed).
These are my Mass Duel sim tests for Offense and Defense. In the "Offense" tests, assume Fir imitated, the enemy attacked back on their turn, and if both are still alive, Fir initiates again and so on. For "Defense", assume this sequence begins with the enemy initiating.
Glacies/Flashing Blade: (Offense): 189-44
Iceberg/Warding Breath: (Offense): 177-56
Glacies/Flashing Blade: (Defense): 193-40
Iceberg/Warding Breath: (Defense): 220-13