I had this idea, a blade and weakness that increases or decreases the amount a blade, shield, trap does. (the blade does not work on itself, so you can't just keep stacking them)
For example, you use the blade that adds 20 to your next trap. If you use ice trap, instead of 30% it will be 50%.
The enemy uses the weakness on you for 25, your feint will be 45% instead.
And as I said you will not be able to use it on itself, so you can't have the blade increase another charm blade.