Imagine you have a toy car made of copper. Copper is a metal, like the insides of your toys and the jewelry your mommy wears.
Now imagine you take this toy car and you hit it with a hammer really, really hard. Again, and again, and again. It starts to get all squished and bent out of shape, right?
Now, imagine instead of a toy car, you have a person's head made of copper. And instead of a toy hammer, you have a really big, heavy hammer. And instead of hitting it a few times, you hit it over and over and over again.
That's what a "copper beaten skull" means - someone hit a person's head with a big hammer so many times that it got all squished and deformed like a beaten-up toy car. It's a really, really violent thing to happen to someone and can be very dangerous.
Luckily, copper beaten skulls aren't very common nowadays, and we use our words to solve problems instead of violence.