Ni No Kuni 2 has 10 Dreamer's Door Locations. Surviving all 10 unique Dreamer's Doors unlocks the Dream Big trophy / achievement.

The first 9 Dreamer's Doors are part of Side Quest 002: Top Marks for Trying. They are all marked on the map. This quest triggers automatically during Chapter 4, it cannot be missed. When the quest starts you also receive the key to the Dreamer's Doors.

None of them are missable! You can still get them after the story in free roam. Some places are unreachable until you've made enough story progress (unlocked Zippelin at start of chapter 7).

Only the 10th maze is well hidden. It's on the island east of Broadleaf called Faraway Isle. On that island is a forest with the door. You get the key by completing the first 9 doors and returning to the quest giver in Goldpaw.

The boss of the 10th Dreamer's Door maze is Level 95! After doing everything else in the game you'll be level 70-75.
Grind to level 95 by replaying the final story boss over and over again. Just lock on to him and press Square to kill him. Then skip all cutscenes, save game and reload from main menu. It took me 6 hours to get from level 70 to 95 with this method but then the last maze boss was really easy.

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#1 - Grotty Grotty - 0:05
#2 - Sundown Woods - 0:50
#3 - Eert Grove - 1:26
#4 - Shrine of Pining - 2:18
#5 - Tidewash Cave - 3:05
#6 - Sublime Shrine - 4:03
#7 - Crooked Cavern - 4:57
#8 - Blowtorch Cave - 6:08
#9 - Shivery Shrine - 6:50
#10 - Faraway Woods - 7:40