You will obtain the key to Eureka after defeating Doga and Unei. The entrance to Eureka lies on the 1st Floor of the Crystal Castle in the room in the middle.
Once in Eureka you will need to defeat bosses to obtain each Weapon.
4th Floor - Moonring Blade - Boss: Amon
5th Floor - Omni Rod - Unguarded
5th Floor - Masamune - Boss: Kunoichi
6th Floor - Excalibur - Boss: General
7th Floor - Elder Staff - Boss: Scylla
7th Floor - Ragnarok - Boss: Guardian
The bosses are fairly easy if you are over level 50. Kunoichi is one to be careful about as she has high agility, attacks three times per round instead of two and her attacks have a chance to inflict poison. Good news is her Defence is not so good.
Make sure you have a White Mage with Curaja and Characters with high attack power. I had two Dragoons and a Dark Knight.