You have 30 seconds to kill 100 Phantom Babies. Go!
There are several clusters of the little insects scattered throughout the room. Remember to equip Ebony & Ivory before you enter, since the bugs die in one hit. Even stepping on them is sufficient to kill them. Start stomping and blasting. Don't worry about killing every last one of them; there's more than 100 in the room. Just start near the door, kill the majority in one cluster, then move to the next, firing all the time. If you still can't do it, a Holy Water will clear the room in less than 1 second.