Ghastly can be placed to reduce the temperature of the room or incite doom. Boo's jangle chains will lure people in to that location.
To do a mass scare use the living room area on the first floor that they all seem to congregate to. To wake him up Bind Boo to his room and Rattle Chains. He just so happens to be sleeping next to him. Wendel is unlocked if you can kick his bully out of the house without him being the last one. Just make sure Aether is placed just outside of the house and singing away. Aether's most powerful siren's scream just so happens to be powerful enough for this. Terroreyes is unlocked by using a scream that can shatter the glass holding him. You can use a pretty broad range of scarers because this level does not have no more particular goals than to simply scare people away.