Only Usopp can defeat Loki. This is one of those cases where Loki's Devil Fruit ability revolves around lies and truths. Usopp is uniquely capable of turning lies into truths, while Loki's ability is about transforming lies into reality. For example, Loki could say, 'I am stronger than you,' and if it’s a lie, his Devil Fruit power would make it true. However, only Usopp can reverse this effect. It’s pretty hilarious when you think about it! It reminds me of Perona, she was also someone who couldn’t be defeated by anyone except Usopp