I think before the CP9 Saga, Usopp would actually stand a chance against Luffy. However, he said himself that things weren’t like they were before and that they’re just too strong compared to him now.
If he has like 500m between him and Luffy, he pulls it off. Eg can fire sea stone at him before Luffy even knows where he is, then blow him up. Pop greens would fuck up Water 7 Luffy.