When you have finished a chapter, or are about to start another one, you will return to the Gontranno Sanctuary courtyard. In the courtyard, there is a combat dummy which you can test your your weapons on. It appears to be invincible, but there is a way to kill it. Go to the shed and take the Fiberwire from the wall. Stand behind to the right of the dummy. You have to stand just barely within reach of it. Crouch by holding L and use the Fiberwire on the dummy by pressing R. If done correctly, the dummy will pop out of the ground, squirm around, then its head will pop.