I reckon the magic martial arts have been largely forgotten, the lich made no exception there, so the MC has a huge advantage. If his opponent doesn't possess that ability at all, he pretty much needs to do nothing but use it to defeat them. Looking at the lich's body, which wasn't a mere skeleton at all, I actually though it had tried to compensate by having a strong, magically enhanced physique, but it clearly was nothing spectacular compared to real magic martial arts.