Reaper

The second-to-last Assassin-type monster, but despite its fearsome appearance, appearance timing and placement in the list, it's actually the weakest monster of that type: its stats are even lower than those of the Skull, and its Stored Attack, Field Effect Reduction, is nothing to be worried about even if it manages to use it, as it reduces the effects of Hyumas by a very brief amount of time. Therefore, even physical attacks or Songs that haven't been charged for a long time should be more than enough to dispose of it.