Hi ! It's not a guide but a little help.This cheevo has popped for me at 411 chests opened and there're more than 470 chests in the game.So you have not to open each of them
And there's the guide i followed for this game.This is the best guide for Pier Solar(Fun fact: developers use it to create their official guide :P ).Thanks to Tim333 for his wonderful work on this.
But CAREFUL !!! This is a guide from the Genesis version,not the HD version.This version contains new dungeons with new chests.Thankfully there's only 2 or 3 new dungeons and the chests are very easy to find.
Here's the link:
http://www.gamefaqs.com/genesis/618165-pier-solar-and-the-gr...
Also the Zellini's magic,"Bright Mind" is very useful to check if you missed a chest in the area you are in.
Good treasure hunt ! ^^