Complex sentence: Inertron is frequently described as indestructible and impenetrable; however, this only applies to regular humans, lesser superhumans and 30th century known technology, as, on some occasions, Superboy and Mon-El could break an inertron container by using all their strength.
Split into simpler sentences: Inertron is frequently described as indestructible and impenetrable, but this apparently applies to regular humans, lesser superhumans and known 30th century technology. On some occasions Superboy and Mon-El could break an inertron container by using all their strength.