@Zero101
Creating/using techniques requires more than just "techniques", it also requires muscle memory and body condition. So if that training really did not affect his body, he wouldn't be able to use any technique.
Also let me write it in number : 100 000 000. That's the number of years he trained, people tend to underestimate how much it is because they are used to big numbers, but 100 million years is a LOT. Even if that girl were to be some kind of 1-of-a-kind prodigy appearing every 1000 years that can already match with top tier adventurers that have been practicing for 60 years nonstop by slaying giant dragons or whatever, that'd still be 1 000 000 times less than MC training, that'd still be 99 999 940 years of training needed before reaching MC level.
Even if, and only if, his body did not improve at all and he only "mastered technique" as you said, he should still be able to predict every single trajectory/every single movement of his opponent right when they start moving. He should be able to evade/counter every attacks without having to block anything with his sword.
TL;DR MC is just horribly bad, or the author doesn't have any sense of practice time.