Shiryuu: Anything sharp that can cut him (swords, blades, knives, you name it) Also, any attack with Haki imbued in it. For example, Garp is always able to hurt Luffy with his punches. Everyone one ignored the simple answers... Water and Seastone (Kairoseki). As for the Lucci and Sengoku hurting Luffy with physical attacks... that is debatable because Lucci was using special abilities from his Rokushiki training and Sengoku may have had extra power on his attack because of his devil fruit. However the true counter to his fruit is in fact any form bladed or spike attack.