The Gremloblin, is a mythical creature and the main antagonist in Gravity Falls episode, "Boss Mabel". He is half of both a gremlin and goblin, who can make people see their worse fears if they look right into his glowing eyes.
According to the Journal, using water against him should only be as a Last revolt, as water only makes him more angry. When he looks in to his reflection, hench looking in to his own eyes, he see their worst fear... Becoming like his Father. This causes the Gremloblin to run back too the Forest, in terror.