Well, I finally found out why Faye is the way she is. She can actually be super powerful with the right build. I was on Reddit looking at the Adult Tiki build thread when someone mentioned her speed wasn't salvageable, and there was no point in it. For some reason, that made me think of something.
Faye (+Attack / -Res) <- This is the best, but +Speed is respectable. Every other bane loses out though.
Firesweep Bow+
Luna
Darting Blow 3
Seal Attack 3
Threaten Speed 3
HP + 3 seal
Without buffs, she will only get KO'd by 2 units, Kagero and Raven. On her second turn, she'll go 92/2/15 (kills, deaths, draws).
Not too impressive, but +4 speed, she now goes 101/1/7, losing only to Raven. If you are running Erika or Ephraim (+3 attack / +4 speed), she goes 105/0/4 on her 3rd turn. Even if every unit is +attack, she still won't die.
Here are a few more stats before I talk more about the build:
+attack, Fury 3 enemies: 100/6/3
+attack, Life or Death 3: 95/12/2
+attack, Death Blow 3: 99/7/3
+speed, Fury 3: 98/6/5
+speed, Life or Death 3: 91/16/2
+speed, Death Blow 3: 100/6/3
So, as to why this works. It turns out that with +10 speed, Faye is actually able to double key threats, assuming they are not +speed (Raven and Kagero are big ones). Thanks to Firesweeping, she doesn't get countered back. As you can see above, she does lose out when the enemies get significantly more speed (most of her loses are from getting doubled). This is important, because if you manage to get threaten speed down on the first turn, she actually prevents a lot of deaths, even if she doesn't get to attack first.
To put this into context, she is actually better than all of the other archers for doing this, because her bulk is actually awesome. Her speed isn't too big of an issue (surprisingly salvageable?), especially if you run an HP battery with her, she can easily kill a large number of units without going down. Of course, doing this requires +attack/-res Faye, so she can take the right hits, and kill the right enemies (mages mostly, which is why her res isn't hugely important). Really makes me hope she ends up as a 4* (or maybe get a 3* version) so I could get a hold of this and give it a try.
This is probably it for looking at the Firesweep bow. I think I've looked into most builds surrounding it at this point, and now that Faye is actually viable (-speed isn't viable, no matter what I've tried with the current skills available) that is the last of the archers who can use it (Niles is terrible still, and there isn't any fixing that).