Don't want to downplay Sommer but with that ability he feels like a perfect Usopp's opponent. Usopp taming his fears would be an amazing development for him on Elbaf. Maybe that's how he becomes a "brave warrior of the seas"
I'm all for Usopp having a great fight, but at the same time, I have a hard time believing Sommers will be his opponent.
The last major fights Usopp had were against Perona, Daruma, and Sugar.
Usually Oda has Usopp fight the weakest member of the group, I really don't think Sommers is the weakest member between him, Gunko, and Killingham.