If a huge laser pointer made a hole in the earth and destroyed the core, how would our planet react? The first question is of course, when the core is an iron-nickel ball with a diameter of a thousand miles, how wide the laser beam must be to "kill the core". The second problem is that when you use a laser to drill or cut something, the removed material must go somewhere. On a small scale, for example, using a laser to cut wood or metal shapes from a sheet that is a fraction of an inch thick, the material is ejected from the hole and dissipated in the air.
However, once you reach a certain depth, your laser will pass through clouds 10 or 20 miles or deeper, full of anything that has evaporated. All of these things are absorbing your beam and heating the surrounding materials until at some point *all* your laser beam is absorbed by the borehole and you are not making more progress in the deeper process. At the same time, all this heat causes the surrounding materials to become hot, soft, and possibly melt and collapse to fill the holes.
Most of the silicon, iron, and other materials ejected from the cave will be cooled enough to fall out of the atmosphere and deposit nearby, and also tend to collapse inward, so basically it will eventually form a volcano that is constantly trying to collapse. Now, you can solve most of the problems by using a beam that is wide enough-but this will push the energy requirements to a crazy level, and there are some problems with the laws of physics.
That is, even the laser beam is not collimated. In other words, they are not completely parallel, but scattered. Much more than you think. It is impossible to make a green laser pointer light from here on the earth, and when it reaches that distance, its width will not be wider than that of the moon. So...you have to figure out a way to deceive the laws of physics, make the beam wide enough and strong enough to penetrate the earth, but *avoid* let the laser itself overheat and self-destruct.
But even if you wave the laser and its power with your hand (and doing so offends the god of magic), you must evaporate *a lot* of rock and make it hot enough to *not* settle down the hole nearby and return, but actually On left this planet. "Many" means "most parts of the earth". The end result is that when your borehole reaches the core, it makes no sense to ask how the "earth" will react, because its response has changed the situation to the point where it is no longer the earth.