So you can get the Zodiac Spear relatively easily. It is the most powerful weapon in the game. But will only show up (garaunteed) in one specific chest if you DO NOT collect four specific different chests during the play through. It helps that none of the four you have to skip have anything useful.

The only other way to get it is if you decide to collect ALL the summons and on the way to the last summon there is a chest that has a 1/10,000 chance of having it if you use the relic that changes the chances to acquire items from chests. To top it off, there is not a save near the chest and some pretty nasty critters, even for high level characters...

So get the hint book...save you some trouble.