Any sub-weapon that you can buy at the shop can be mastered at a price. Buying the item again earns you 10 percent towards its mastery but this does not apply to sub-weapons dropped by enemies.

To master all sub-weapons quickly (except the mastery ring) go to the lowest portal in Forgotten City on the the right hand side of the map. There is a save point on the right and a room with mummies nearby. When you run out of MP, simply go two rooms right and use the save point to fully restore your sub-weapon MP.