With Calva being one of the champions and inhabitants of the red space, he was naturally strong. Each champion's strength was different as it played to their strong parts. One factor about all of them was their physical strength.
Their bodies, in terms of endurance, speed, and power, were all at the highest levels. They truly felt that they weren't lesser than anyone out there. This was also true for Calva's case.
He was stronger and faster than any Skully and had power to compete with the demon kings. One of his natural traits was the special bone he could produce from his body. An almost unlimited supply, and at the same time, it was harder, and could be made sharper than any of the Skullys.
Even an amateur, if they were to grab a bone sword made from Calva's body, could cut a diamond, and even the hard red crystals of the Durum race. Yet right now, his opponent had grabbed his spear like it was nothing.