Nico Robin has hand powers, that could defeat Usopp. Usopp has a bow, but it won’t be able to win. Usopp is the sniper king and will keep trying until he ends his part of the arc. Nico Robin wins.
In this site’s poll, why is Nico Robin always beaten up by another strawhat crew? How come she was even overpowered by Usopp/Nami/Chopper? (You can look it in another poll in this site)
I wouldn’t say she’s the weakest member. Usopp is weaker among others. I am a bit surprised that She’s losing in the polls. Maybe it’s only a temporary thing 🙂
DReager1.com 
In this site’s poll, why is Nico Robin always beaten up by another strawhat crew? How come she was even overpowered by Usopp/Nami/Chopper? (You can look it in another poll in this site)
Is Nico Robin really the weakest Strawhat crew?
I wouldn’t say she’s the weakest member. Usopp is weaker among others. I am a bit surprised that She’s losing in the polls. Maybe it’s only a temporary thing 🙂