Did you pay attention to the tutorial text in your first fight? Seems to me that you don't have a very good grasp of the game mechanics.

Reading through this page might help:...

You play it like any other card; pay its casting cost.

You must equip (play) a weapon in order to deal damage with your Heroes. (Unless they have a damage dealing ability i.e. Nishaven, Zaladar etc.)

Weapon cards usable by Gwenneth are: Soul Seeker,...