There are so many badass moments of Zoro, I forgot about this one. He was ready to straight up, cut his legs off(halfway did it) just to get free. And when Nami asked what would he be able to do next, he's just like, "i'll just defeat them". His confidence and logic is summarized as, "if i can swing my sword at them, I'll win".
Not only that, but he pep talks the giant.
"hey, man. Imma cut off my legs real quick so i can fight these guys. You wanna tear through the swords jammed into your limbs and join me? Better than just dying to these cheap ass mofos, right?"