Open all treasure chests in the game except the following:

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 sixteen chests all near each other. The fourth chest among all sixteen must not be opened, so it is best not to open any of them. They only contain Gil.

Then, go to the Necrohol of Nabudis to find the most powerful weapon in the game, the Zodiac Spear. It has a +150 Attack with +8 Evasion.