The town's and castles needed are as follows:
Cornelia
Castle Cornelia
Pravoka
Elfheim
Eleven Castle
Mount Duergar
Melmond
Crescent Lake
Gaia
Onrac
Lufenia
That's every monster free town and castle. If that doesn't work try these secondary locations, I had already visited them during the course of the story, so no telling if they are needed as well.
Caravan
Sage's Cave
Dragon Caves
Matoya's Cave
Western Keep(There is a boss here early on, but otherwise "monster free".)