Cave of Mysidia (FF2DoS)

I already used both masks so far (one on the statue in the city and one to make the doppleganger go away) and I searched from top to bottom of that dungeon in the Cave. What exactly am I looking for? The walkthroughs at game FAQs are NOT helping. Can somebody maybe show me some maps of that dungeon or provide some info?

At the bottom of that cave their should be a crystal staff you need to acquire. Sorry I can't provide a map.

I noticed four tresture chests next to eachother but I have no clue how to get to them.