Something being hard or difficult is one thing but when asking what is harder then that is something else. As Riku's Twilight said Demyx was harder then Xemnas as well as almost every other boss. Honestly the Xemnas fight was a little dissapointing. Preporation is very key in my opinon, that is why to me Demyx is hard in proud mode when you don't use summons or are at an improper level then yeah he's hard, especially when you're trying to rush the game and see how fast you can finish. However in standard mode, prepared, without summons again, at whatever level I was then, no he's actually easy, beat 'em first try.