When you defeat some monsters they will drop loot. Always try and pick it up, although you may never use it you will be able to sell it for coins, especially if it is weaker than what your character already has. You can tell if it is weaker by looking to see if there is a red arrow pointing downward to the right of the weapon or looking at the item's number which will indicate whether it is inferior to what the character presently carries.
Always try and make sure that your character is equipped with the best weapons available at all times. This can be done by either switching or uprading the current weapon. You may have to part with some gems for 100 percent guarantee unless you have an Element in your inventory as just spending coins alone may not successfully upgrade a weapon.
To make your character more effective in a fight you must always look to upgrade their Dash, Block and Charge maneuvers. Each of the moves features upgrades spread across three categories (Special, Damage, Rage) that help transform your character into a more efficient killing machine. To upgrade a character you will need coins or gems and as you progress through the game the upgrades will get more expensive.