The theory behind it is that you render yourself invisible, either before or during the fight. This means you shouldn't be tracked too easily by anyone and they can't listen for you either thanks to the slumbering dragon ring's effect.
The heavy crossbow is for those who try run away from you and whom you cannot reach for any reason, might also allow you keep safe distance away from your target who is close to finishing his/her heal.
Just be wary that you WILL be immobilized after each shot until you either manage to reload or get hit.
The rest of the levels are dumped into vitality so you have something to fall back on if you run out of casts.