TL;DR Faye isn't very good (at least with Firesweep +), Setsuna is the most efficient Firesweeper (considering ease of getting skills) and If you really want to do this, Takumi with an Erika buff battery and Life or Death 3 will be the most effective (65 kills to 1 death, love Reinhardt). There may be some mistakes, but nothing that would significantly change the results.
So, I was doing some more looking into Faye and Firesweep. Turns out, Faye is a straight up a terrible archer, especially since I rolled a +speed/-HP Rebecca today. Firesweep can be a little more interesting though.
So, my actual findings. First off, since Faye isn't in the sim yet, I used Rebecca as a base and gave her the following:
Rebecca - +hp(Easy enough to tell if something is off) - speed
Buffs: +1 attack, +5 defense, +4 resistance
Enemy buffs: +9 speed, to make up the difference between Faye and Rebecca (+10 if we want to test -speed Faye)
Weapon: Killer Bow+ (11 damage to match the Firesweep, and the doubles dagger users thing isn't too big of a deal, we can just remove those)
So, I'm comparing Faye, Rebecca, Setsuna, Takumi and Jeorge. I've also removed all skills from them for our baseline. They will all be using the same load out:
Weapon: Killer Bow + with a +2 attack buff to match Firesweep. Chalanger only attacking, and we'll ignore any loses, since with Firesweep, there would be a counter anyway. Oh yeah, neutral stat lines.
Kill ranks: Takumi = Setsuna > Jeorge > Rebecca > Faye
Kill count: 14, 13, 10, 2
Now onto the more fun stuff. After looking around, they best skill for our pseudo Firesweep user is Deathblow if we only consider them attacking. This pushes the kill ranking to:
Kill ranks: Takumi > Jeorge > Setsuna > Rebecca > Faye
Kill count: 36, 33, 32, 27, 15
Still, not too surprising. Now, a key part of Firesweeping is being able to A) Take the incoming hit or B)get out of the way of an income hit. I'm just going to assume that we have to take A, because you have better units to dance. We also need to use our B slot for something, so we'll choose Seal Attack 3. This effectively gives us +7 def/res after we attack unless the enemy has Harsh Command. This will also balance out Life or death, effectively giving them +2 def/res instead. So, I gave everyone an additional +7 def/res and set it to the enemy attack. We'll just ignore the wins. None of the wins could double our targets anyway.
Current Build: Killer Bow +, Life or Death 3, Seal Attack 3
Buffs: +2 attack, +7def/res to simulate after they had been attacked
Death Rank: Jeorge > Takumi > Setsuna > Faye > Rebecca
Death Count: 0, 1, 6, 7, 8
So Faye is out of the gutter finally, but she only surives 1 extra unit. Its nice to note that they lose to completely different units though. Rebecca loses to Klein, Ogma, Effie, Raven, Kagero, Cordelia, Donnel, and Hinoka while Faye loses to Raven, Minerva, Corrin M, Ryoma, Hana, Lucina, Sharena. Setsuna loses to Rebecca's list, minus Raven and Kagero, and Takumi only loses to Reinhardt.
So, even though Faye survives more than Rebecca, she loses to fairly commonly used units, actually making her worse than Rebecca still.
So one last test, assuming we want our Firesweeper to balance being bait and killing things. For this, we'll walk away from Life or Death and look for the next best skill for killing. In this case, we get the following:
Faye: Death Blow
Everyone else: Swift Sparrow
Kill ranks: Takumi > Setsuna > Jeorge > Rebecca > Faye
Kill count: 30, 27, 26, 23, 12
Death Ranks: Setsuna, Rebecca, Jeorge, Takumi, Faye
Death Count: 0, 1, 3, 4, 5
Right, so that is interesting. Setsuna is now our best Firesweeper if we don't use Life or Death, followed by Rebecca. Problem is, Swift Sparrow isn't avaiable anymore, and it is hard to recommend sacking Spring Lucina for it. We should note though that overall, Life or Death Jeorge is our best user, and Swift Sparrow Setsuna is in second.
Lets try to make this more affordable. Ignoreing both Swift Sparrow and Life or Death (both fairly hard to get skills) how do they stack up?
Rebecca and Jeorge: Death Blow
Setsuna: Fury 3
Takumi: Darting Blow
Kill ranks: Takumi > Setsuna = Jeorge > Rebecca > Faye
Kill count: 27, 24, 20, 12
Death Ranks: Setsuna, Rebecca, Jeorge, Takumi, Faye
Death Count: 0, 1, 3, 4, 5
Our death ranks/count don't actually change here. So now, Takumi gets the most kills, but Setsuna is still blocking the whole world.
By the way, If you give an Eriak brand battery (+3 attack, +4 speed), everyone but Jeorge and Faye survive an enemy attack (lose to Hana / Hana and Minerva). Rebecca is only 1 kill off from Setsuna, and Takumi is king again with 45 kills and 0 deaths.
So to wrap this all up, Without buffs, Setsuna will get you the most mileage out of the Firesweeper, only because she is the easiest to build, and will always surive at least 1 round of combat if she initiated on the target. If you have spare Life or Death fodder and Jeorge, you can make a better Firesweeper, but I'm not sure it is worth the effort. Finally, Takumi is better off ignoring this whole mess, and Faye is trash. Well, not complete trash, since she gets more kills that Virion and Niles, and actually Niles dies more than her while Virion is slightly better, but the quality of competition is fairly telling.
There may be some mistakes, but nothing that would significantly change the results.
TL;DR Faye isn't very good (at least with Firesweep +), Setsuna is the most efficient Firesweeper (considering ease of getting skills) and If you really want to do this, Takumi with an Erika buff battery and Life or Death 3 will be the most effective (65 kills to 1 death, love Reinhardt).