Arms Master Achievement
Bring all party members' weapons to their max. levelTo fully max out your weapons and armor you will need to do two things. First, you need to purchase the new forms of the items at the respective shops, which will increase them to S, SS, SSS, and Max. You'll need to complete various quests for the shop owner to build up the stock, so keep checking for those side-quests as you progress through the story.
The second aspect is to increase the rarity of the item at the Smithy. Each upgrade from the actual shop will set a new cap for how far you can increase the rarity at the smithy, and each upgrade to the smithy itself by completing side-quests will have an effect as well. Once you Max the item at the shops and then purchase every upgrade at the smithy to get to +30 will finally have them maxed out completely.
You can see this guide for a brief overview of where to get the various resources you'll need for these upgrades. You can equip accessories to help during your farming depending on what you're after. If you're mining, put on ones that increase rarity or number of items. If you're killing monsters, put on ones that increase drop rate and rarity. Remember to not sell any of the precious gems as I mentioned in the Road Map, as this step can take much longer if you use those for a quick cash grab early and then need to replace them later on.