Hidden walls are found all throughout this game but for this achievement, it doesn’t necessarily have to lead to another room. I unlocked this breaking some sort of structure that led to a upgrade (health I believe).
Here’s three breakable walls early on in the Galleon Minerva:
- After you head outside into the rain drenched deck of the ship, there are a couple of respawning Seamas and a Aello floating near the broken mast. Jump on the mast from the right and go all the way left jumping to an area above where you entered from. At the far left passed the chest is a breakable wall with an upgrade in it.
- The second one I encountered is below the first save room at the bottom of the vertical room with a couple Seamas. Attack the wall on the right and you can enter to another room.
- A third can also be found on the adjacent vertical room to the left, dropping below where a blue chest is. Break the wall on the left for an MP upgrade.