You know you can get it quite early in the game, just run away from alot of the mobs and make a beeline straight for it as soon as it is accessible, till then just ignore the chests, there isn't alot of stuff you cant get in a shop, not to mention even if you do get it you may not even have the licence for it anyhow, and again who is to say it will really make all that difference when it comes you current weapons, the Zodiak Spear is just a truly awesome weapon and it was so gratifying that I forgo the required chests and get the Zodiak Spear... the damage it wields is simply staggering, even Penelo can hit for 9999 with the weapon.