Wikia

These are monsters in the Golden Sun games whose one lowest resistance rating is the Mercury element, meaning that using Mercury-based offenses against these enemies yields better results (this is indicated by three "!!!"s at the end of each Mercury attack). This is important to the Dark Panther method because when an offensive Mercury Djinni is used to slay any of these enemies, that enemy will flash colors before dying and you get increased EXP and coin loot, and the chance that enemy will drop its held item will increase by four times.