Wait, he knew how to fight somewhat from the beginning, but was hindered by the physical limitations of his new body. Now he knows a masterful martial art, but he spent the majority of his time plagiarizing an Earth web novel. Even if he devoted time off-scene to personal training, he shouldn't have been able to raise his physical ability to match that of his new martial art.
He should be suffering from physical strain as a result of pushing his body the way he has. This entire fight is a giant plot hole.