From the plugin's Introduction and Instructions:
"Not everybody wants NPC's to travel all over the place. With this plugin,
you can set NPC's to be unable to move pass tiles marked by a specified
Region ID. Simply draw out the area you want to enclose NPC's in on and
they'll be unable to move past it unless they have Through on. Likewise,
there are regions that you can prevent the player from moving onto, too!"
Try setting "Through" ON for the boss event. (It's in the Options box in the bottom-left corner of the Event page.)