Step 1 - Main Story
Your first step in Ni no Kuni II should be to play through the main story to the very end. There are no missable trophies in the game, so you don't really have to worry about anything as you go through the game. However, you should do the sidequests as they unlock, as not only will this contribute to those related trophies, but you will also be working on building up your kingdom. There is a roadblock where you have to level it up to three during the main story, so you don't really have a choice in working on it at some point. You will also want to work on finishing as many skirmishes as you can, since it can be annoying trying to get some to spawn later.
Step 2 - Post-Game
Once you finish the main story, you will reach the post-game, where you will want to concentrate on finishing up the sidequests, particularly those that reward you with new citizens for Evermore. You will need to recruit all of them to level your kingdom to four and if you've gotten them all prior to finishing the story, then you will only have four left in the post-game. In addition to finishing off your kingdom (go after the influence trophy here), you will also want to go after the collectibles and Tainted Monsters in this step. Do the higgledy stones and Songbooks before the Tainted Monsters, as more Tainted Monsters will spawn after the first 50 are killed (you don't have to do anything with those).
Step 3 - Dreamer's Mazes
You should have already finished the first nine mazes in the previous step, since one of the citizens is locked behind a sidequest that involves you clearing them. This will allow you to go after the 10th, and final, Dreamer's Maze, on Faraway Isle. Now, this one is 30 floors long and the boss at the end is level 95, so you might want to grind some. You have three options here: the final boss, the Kingmakers refights, or Horraura, Tainted Monster #57. The last option is the best and quickest, so once you feel you are at a sufficient level, make your way through Faraway Forest Maze and finish off the boss to complete it.
Step 4 - Cleaning Up
This final step will have you cleaning up whatever else you have left to do, such as the miscellaneous trophies. It might include accruing the necessary guilders, cooking dishes/higgledies, crafting weapons/armor, finishing off any skirmishes, and more. Once you're done with everything, you will have yourself another shiny platinum trophy.