The first bosses we found really easy. But the Mountain boss was the first one that took us awhile, I think we went through 4 or 5 whole mountain areas before we found where the boss was. So far same thing for the Plains boss to (we have checked about 7). First 3 bosses we found pretty easy but these last two took awhile lol. Just check every area of the mountain biomes and you should find it at some point, its different for each seed/world where they are located on the map. Good luck!