Setsuna, Absent Archer
Setsuna is THE Brave Bow Desperation archer. She is not likely to kill everything in four hits, but she (and YS!Gaius) is the most likely out of all the archers to land all four hits.
Unlike Blade mages, Brave Bow archers can operate more independently from their teammates. Additionally, being colorless means that Brave Bow archers are equally effective against all types units and are not hindered by the Weapon Triangle. However, as far as archers go, Setsuna has one of the weakest Attack, beaten only by Niles. Sestuna is pretty reliant on her Special to deal damage.
While Brave Bow archers are a little more versatile than Blade mages in terms of coverage, they have problems dealing with certain units that Blade mages can easily deal with. Bulky Distant Counter units are quite common in the upper tiers of the Arena and they are dangerous for Brave Bow archers to take down, and Setsuna's pitiful Attack and flimsy bulk makes this extremely apparent. Raven mages are less frequently encountered, but they still appear often enough that players should be cautious and double check enemies' skills; some mages that usually run Blade builds may occasionally run Raven builds, as the sole purpose of those builds is to make Brave Bow archers disappear under unsuspecting players' fingers. Like all other Player Phase nukes, the greatest threat comes from ranged cavalry who can cover large distances quickly and take advantage of Player Phase nukes' weak Enemy Phase performance.
As a nuke, Sestuna will do fine against fliers and other flimsy glass cannons. Just make sure she avoids anything bulky that can counter attack.
Level 40 stats:
HP: 34/37/41
Atk: 24/28/31
Spd: 34/37/40
Def: 19/22/25
Res: 20/23/26
Total: 146~148
Default skills:
Weapon: Assassin's Bow+
Assist: Reciprocal Aid
Special: [none]
Passive A: HP +5
Passive B: Bowbreaker 3
Passive C: [none]
Brave Bow:
General use, Arena Assault
Firesweep Bow:
Arena Assault