Old friend
Nero knew that Arter's sword was extremely sharp, and although he had not been able to observe him properly, he understood the gist of his fighting style. It would take him some time to use an attack strong enough to cut through berserker bone, though for himself even a weak attack would be lethal. It wasn't like Nero's body, or his defence, were on par with the berserker.
Having said that, his spear should be fine. After all, metal was metal, and could not be compared to bone.
Indeed, he blocked the attack successfully, but was shocked by the mistake he made. Literally. The blue light that coated the edge of his blade was the result of Arter's ability which carried an electric charge.