Brave bow for this Lyn?

Just use the normal bow is pretty good.

Agreed. Her standard set is pretty amazing as is. If you ever get a +Atk one, then consider a brave bow build. :) but otherwise, Mulagir is awesome as is.

If you want to clear some maps Pretty easy go for the brave bow+.
If it's for arena then mulagir or the firesweep are better.

At the higher tiers in arena/AA, you'd want to invest in a Firesweep Bow due to how common dragons and armour units are; the dragons will bypass Sacae's blessing and counterattack, whereas for amours most of the time brave bow tends to just feed special cooldowns while barely scratching through their def.

If you give her Summoner Support and buff her with Hone Cavalry she'll hit 42 Speed on initiation. That will quad quite a few units. With Luna, she'll do fantastic work, even on Armor Units. I use mine mainly for TT, with Priscilla providing heals and Hone Cavalry. I don't run her in Arena because her point value is so much less, due to lower BST than my main units and being +0.