The most powerful weapon in the game is actually very easy to unlock if you do things right from the beginning of the game.
The key to unlocking this weapon is avoiding opening certain chests periodically throughout the game. If you avoid them all you will be rewarded with the spear near the end of the game at the Necrohol of Nabudis.
The 4 places with treasure chests that must NOT be opened in order for the Zodiac Spear to appear:
1) Treasure chest in Lowtown, outside of Old Dylan’s place
2) When sneaking into the palace (Before getting the Goddess Tear), there are several chests in the Cellar. Do not open the two chests in the southeast corner.
3) All treasure chests in the Confiscatory (the place where you get your weapons and armor back after being captured)
4) In the Phon Coast, later in the game, there is an island with 16 chests all near each other. The 4th chest that must NOT be opened is among these, so it is best to leave them all alone. They only contain Gil anyway.
If you’ve left all these treasures, you’ll find the Zodiac Spear in the Necrohol of Nabudis. It sports a whopping +150 attack and, like all spears, +8 Evasion.