Does Duramboros resist water or something? I spent about a half hour whacking at him with my Ludroth Dual Blades and he didn't go down. I mean, because it was so easy to get in on him and his attack pattern, I was hitting him pretty much nonstop, so all I can figure is he was resisting my attacks or he has an ungodly amount of hit points. Either way, he ended up just BARELY winning the war of attrition (he was limping for a good 5-10 minutes before he got me).
He takes the least amount of damage from water element.