Nature has already provided blueprints for biological changes to support head bashing techniques, just look to goats and woodpeckers for how they can avoid giving themselves headaches.
Both of these species have incredibly dense bone in the area of the skull called the braincase. Additionally because you wouldn't want pokey bits of bone skewering your brain when you intentionally bash your head against something, goats and wood peckers have a very smooth bone around their braincases. You will also want to limit the amount of room the brain can be moved inside the braincase, these animals do so by having less cerebral fluid than us humans.
Lastly, whenever possible if you can restrict the direction of force and reinforce that vector you will be better off for it. Being a cyclops and having just one eye makes this slightly more challenging as you will want to ensure a head on head butt to avoid having the horn glance and twist to the side as this would produce large amounts of torsion on the neck. Perhaps evolution has given your cyclops a sort of tucked position where its shoulders raise high enough around the neck to help protect it from twisting and relies on generating force for the head butt through the rest of its body.
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