Truly is a bit harder than expected. From experience I'd say Medium is really easy just until the last stage. The thing that makes Medium a bit difficult is the fact that 4 random mini bosses assist the reaper, along with the fact that the mini bosses constantly respawn after death.
Sure you can kill them off and use the time reaper is alone to your own advantage, but it's still an annoyance when they come back.
Now really, Hard difficulty now
... I've helped a party beat it before, and I must say.. I'm not sure they'll make another difficulty since Hard already seems a bit overboard. As most people say, even with a high level party, it seems to be a difficulty that cannot be accomplished without spending gold coins, and large sum at that