Well, i'm a balance grand and i cant win the grand hat for some reason. i always win his grand death boots; i know a death that fought him 56 times before he got them. just keep fighting him, you're bound to win them :)
From the best of my knoweldge, it is a very rare drop, so the chances are extremely low. Also, if you are trying to get them on a death character, it seems that school drops a more rare than out of school drops. For example, to get my grand stuff for my death, it took total about 110 battles to get it all. Now, when using my DEATH to get my FIRE grand gear, it has taken about 10 and i only need a dagger. I never used a drop box. Also, my fire guy has been getting extremly good death drops, but rarely a fire drop, so if you have any alts, break those out and farm with those, may help.
