i think i might know which one that is, i cant read korean though so bear with me xD
is that a 7-2 quest? the one with the harpy lady thing? if it is then its most likely the mid boss quest. there are two midbosses, a gold one and a black one. they appear at random during a dungeon so you just have to keep running until you get the other.
if its not a 7-2 quest then idk xD sry