Nier Replicant's weapons don't need to all be upgrading if you want to see all of its endings after B, but you will have to fully upgrade each weapon to work towards 100% achievement or trophy completion. In order to get your weapons completely upgraded, you'll need to collect a long list of raw materials.
To finish the task, you'll need to collect enough Black Pearls to max out your arsenal - thankfully, this is one raw material not required by any quests. Since this one isn't farmed from Shades, you can't rely on words to increase the drop rate, so farming for these can be a little more annoying.
Before you begin, we recommend checking out our full weapon upgrade material guide, so you can make sure you're collecting everything you need from the area. You'll need 15 Black Pearls to complete your upgrades.
Where To Find Black Pearl
Seafront
To begin your grind for 15 Black Pearls, travel to Seafront by either crossing the Southern Plains or ferry. You'll need to be in the second half of the game to find Black Pearls and have already completed the major story quest that takes place here.
There are two locations that Black Pearls can be in, both on the beach. The first you'll find in the area highlighted above, just run along the beach and look for the white glowing item indicators. Black Pearls are very rare, so don't be surprised if you find a ton of Empty Bottles.
The second location for Black Pearls is tucked away in the smaller corner highlighted above, closer to the buildings in Seafront. There a few points between the beaches you can find Black Pearls.
After you've inspected both areas, you'll need to respawn these items again to search for them along the beach. You can enter and exit Seafront to the Southern Plains, or you can save and reload. Do this until you've got your 15, and then you're good to go.