Clash of the Titans

The very first sub-weapon you acquire the Skull Hammer. Is very easy to upgrade and has a good Jump+Ground Pound attack. So it's worth upgrading this one all the way.

You'll need Hammer Exp (gained by sub-weapon seizing hammer enemies) and Gifts, gained from quests, breaking armour and disarming. Enemies that use Hammers will drop the items needed to upgrade the hammer.

Each weapon needs different exp, so if you want to upgrade the sword, you need to sub-weapon seize sword enemies.