If you're using an off-hand weapon that doesn't grant an attack, but grants a defense or attack bonus (like a shield or spellbook), you might also want to consider upgrades.
Adding +1 to your equipment's Defense (Def) or Attack (Atk) costs 20 Nexus Shards (NX).
However, it is not possible to add new bonuses to your equipment, you can only improve existing bonuses.
For example, if a shield only has a +2 Def PHY bonus, it is not possible to improve the shield by adding +1 Def MAG.