You need to have the celestial mirror to open any chests that have the superduper weapons in them. You have the Moon Sigil, so you already know where Remiem Temple is... but if you go to the sides of the temple, you'll see that there's a chocobo race. You have to win that race and you'll get the cloudy mirror. After that you take that to Macalania Woods to charge it up in front of this big round sphere-like statue and you'll get the Celestial Mirror. Then, with that mirror, you can open the chests to the weapons. I think it's on the walkthrough.
