Starting from 435 A.D., a race of sea-faring giants has made land-fall in Eurasia and is raising hell. Due to socio-political pressures in their island homelands in Polynesia and Australia (constant warfare over limited land, internal displacement, overpopulation, etc.), these giants have emigrated west en masse, immediately coming into conflict with the human populations. In a first wave of migration between 435 A.D. and 460 A.D., around 50,000 giants trickle into Indonesia and South East Asia in bands numbering 100 - 1000 in size. In a far larger second wave, 500,000 giants land across the Indian subcontinent and East Asia as far north as the Korean peninsula per year from the earth 500's to the late 900's A.D.. They are organized into kin group bands of mostly young men (~60%) ranging anywhere from 3,000 - 7,500 strong, with the very largest being 30,000 strong. These giants are physically superior to H. sapiens in every way, being
~9ft tall on average
having a bone strength 6 times that of a human being
double muscled making them disproportionately stronger than humans, and in all 7 times stronger than a human adult
50% faster than a human, being able to run 25 - 35 mph on average
acute senses of hearing, smell, and taste, being 3 - 5 times that of the average human
cranial shock absorption in the form of tissue lining the interior of the skull, preventing falls from causing concussions or brain damage
This comes along with a host of giant-specific mutations that negate the negative effects of gigantism.
These giants also come from a warrior culture in which marshal prowess is the acme of human virtue, and thus have highly refined skills of archery, slinging, wrestling, boxing, melee combat, etc. drilled into them from an early age. Worst of all for humanity, the reproductive potential of these giants is several times higher than that of man, meaning a greater capacity to replace numbers and occupy conquered territory.
Luckily for man, these giants have no large domestic animals (dogs, cats, horse, oxen etc.), and are late adopters of iron technology, making their iron implements more rudimentary than the invaded population. However, their arms are by no means primitive, and can be largely modeled after ancient Polynesian weapons.
And unluckily for man, the migrations/invasions occur around the first millennium, meaning no guns or cannons shall be aiding mankind. On top of this, giants have great prejudice against people below 7 ft tall, perceiving them to be unfit, and thus are inclined to enslave and massacre any human populations they conquer.
Ideally these giants would replace some human populations and stalemate others, but given the physical superiority of these beings, it is entirely likely that they may entirely outcompete mankind, doing to us what we did to Neanderthals.
Question Is there anything that would or could prevent humanity from being totally destroyed by giants?