Umm...all I can say is that the treasure chest that contains the Zodiac Spear only appears in the Necrohalls (can't remember the full name), and it'll only appear if you haven't opened certain chests found throughout the game, though I don't know where the treasure chests that can't be opened are...
Dunno the rest of the best (I have no clue how to get them anyway, except for the best katana...well...that's a no-brainer).
I'd say have Ashe use Guns, but it's all up to you.
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