I intended to add a section about this to my Guides long ago, but it was only when I forgot to follow my own advice today that I remembered this point.
In the 2nd Generation, once you have defeated Fiersome and obtained access to the four Underground Shrines in each of the four original Seasonal Caves/Dungeons on the Map, you no longer will be restricted to the usual locations or time of day when searching for a specific War Trophy. If you remember the types of Monsters that you face in each of the Underground Shrines, you can fight the Monsters there for War Trophies that you need irrespective of the time of day or night. Almost every War Trophy is to be found in one of the four Underground Shrines. Time does NOT pass while fighting there and instead of being forced to wait until day or night arrives in order to chase down the Monster type that you need, you can go to the Underground Shrine instead.
Fighting Monsters in the Underground Shrines will increase your Levels as well. Energy Levels are restored at one of the two little altars on either side of each entrance. You therefore have a 50/50 chance each time you enter a new chamber of having all of your HP and RP restored.