I have tried using this on several occasions and its a piece of crap. I have gotten headshots on a bunch of different enemy types and the crossbow seems to do almost no damage at all to them. What is the point in this thing if it is so weak. Granted it is useful under water, but other than that its a piece of crap. Is there a way to make it stronger so it can be a little more use in combat?
I think they made the crossbow crap for the very reason you are trying to use it for. The focus for a witcher is with sword and melee combat. All of the spells are better geared toward combat like this too. If you could sneak around the place taking out enemies with crossbow headshots it would take away from the focus, which is melee combat. So to give a direct answer to the question, you can’t make the crossbow more powerful. It is a piece of crap because they intended for it to be this way.
The main use for the crossbow in the Witcher 3 is to hit underwater enemies or to knock flying enemies out of the sky, which is why it was given to you before the griffen fight early on in the game.