Giriko isn't just a stupid chainsaw
Giriko states that he passed his soul down through his offspring which he implied he made stronger each generation. This means he was selectively breeding for stronger and healthier children to take over with his own soul. Because he would have been an adult (or at the least, a teenager) before making the giant golem Arachne’s soul was hiding in, this meant that sometime around 1204, he had already figured out the basics of modern genetics, over six hundred and fifty years before Gregor Mendell did.
That’s in addition to him not only being the first Demon Weapon, but how he was also the inventor of golems, and staying hidden from the DWMA for 800 years. During those 800 years he became a master of hand to hand combat and incredibly competent fighter even against those trained to specifically deal with threats like him all while protecting Lady Arachne’s soul in the golem from unavoidable threats like plague and the World Wars that wrecked havoc on and around the area they lived in.
While he’s a violent drunk, misogynistic pig, and not the most clever socially, Giriko is a fucking genius.