I think counter attacking the magma/ deflecting is easier than trying to fullbody haki tank it, spreading your haki thin like that is for sloppy scrubs like Pica (and to a lesser extent Vergo) with insufficient CoO.
Even people like Vista can slash at him without his sword melting, and even Jinbe could block without permenant hand damage, so you don't need yonko tier skills to avoid damage to your weapons/ fists.