Maybe you can have the enemies spawn from far away, path them to the house.
Is there not a script to make players enter combat when they enter a box/poly trigger? (set it to a timer perhaps, while the enemies are running for the house) I don't know if there is a call to initiate combat without enemies around though.
Or, you could hide a hostile rat somewhere that initializes combat, rat immediately casts invis on himself while the enemies run to the house...
