Brave Bow Brave Lyn

Bow Breaker, TA, Raven tome, Inigo/Robin/Sophia/Cecilia.

Probably Shigure too.

Dragons with Lightning Breath and TA/Raven users are the standard counters. Certain archers, like Gordin and Leon, can fit the bill.

Look into the new upgraded Deflect Missiles and Distant Defense seals as well. These really open the possibilities.

I've given up baiting them because it's really risky and it's rare that the bait can survive if another unit comes into range. Therefore, I have decided that rather than bait+counter, I'll just reposition/dance a unit that can OHKO her before she moves... and then kill/survive whatever else is in range.

See you at arena!

When does Brave Lyn become a problem???, Get a Cecilia with TA, bow break, check both Brave Lyn and Rein at the same time. My arena team consists with Rein and Cecilia, once Cecilia clear these two, Rein can easily rekt the rest.
Just for those Brave Lyn users, get a fire sweep bow and cancel affinity if you really want to troll.
Every time I saw a brave bow lyn with default B slot skill, i thought oh man, another easy kill from my Cecilia or Inigo.

Enemy phase Raven TA Bowbreaker/QR. M and F Robin run this best. F Robin can check both Rein and Blyn simultaneously regardless of buffs and specials and all that, if you give her a Distance Def 3 seal. That's honestly a bit of a broken seal. F Robin uses it well and destroys these two, as long as the Blyn isn't running Cancel Affinity 3. But she's not good for much else right now, as she doesn't have the speed or power to one-shot most enemies. Player phase a Blyn of your own. She can one-shot herself with the Mulagir build and a Hone Cav. Or a strong enough Rein can do similar. Even a melee unit with enough attack can one-shot her if you can space and bait effectively.

Leon