Usopp Vs Luffy Full Fight. Well ok lets put this into comparison. Luffy wins Medium difficulty.
If it werent for Luffys. The only way the difficult could be any different is if Luffy gets close enough to land a hit and if he does it will be all over for Usopp. While he did win Usopp could have easily won had Luffy been blown into the sea by the explosion.
Luffy wins Medium difficulty.
Post Time Skip Luffy literally one shotted one. When I first saw that Luffy would fight Usopp I expected it to last 2 minutes tops with Luffy winning. It took the strongest versions of pre time skip luffy zoro and sanji to even defeat a pacifista. It because of pride insecurities and self doubt on Ussops sidethis was due to the fact that the monster trio were growing in strength and the gap between them and the weak trio was just increasing very fast keeping up was just not possible and Ussop lost confidence in his ability.