The reference to the square-cube law is probably not apropos for a supertall but "healthy" human being. The reference to Robert Wadlow points to an unfortunate gentleman who looks rather skinny, weighed 399 pounds, and was 8 feet 11.1 inches tall. His body mass index (BMI) was 27, making that skinny-looking person actually overweight. A "healthy" weight for Wadlow might be 330 pounds, for a BMI of 20. In mks units, his height of 2.72 meters and a weight of 148 kg. He would appear to be even skinnier.
And the mkS
gives us a clue for survival - if our giant's metabolism is reduced, he can breath and pump blood at normal velocities. He would appear to move much slower - even Ray Harryhausen's movie giants got that part right.
Yes, bone strength would be a problem - our giant might not be able to stand up, certainly not jump. Blood pressure could be another problem; healthy diastolic blood pressure (the lower number) is 60 psi (400kPa) while the gravity gradient for water is 23 feet per 10 psi ( 1 meter per 10 kPa ), so a 12 meter (39 foot) giant would have 17 psi difference between head and feet. Still, people manage to survive with diastolic blood pressures of 80 or worse. Again, our giant will do better sitting down.
With a BMI of 20, a 12 meter giant will weigh 2880 kg. If their brain size is the same proportion of body mass as us midgets, it might weigh 10 to 20 times as much as our midget brains. Again, their metabolism will be lower, but imagine how many synaptic connections that brain could make! We may be talking hypergenius
So, by the time the giant reaches 20 years old, they will have invented antigravity and moved into space, where their weight and bones are no longer a problem. They can continue to grow larger and larger and smarter and smarter, outwitting the rest of us in the competition for food and resources, eventually consuming all the carbon in the solar system, until their antigravity system fails and they collapse into a mole planet
. A disaster unanticipated by any B grade horror movie, AFAIK.