He's not self taught He was trained by a former knight and by an order of assassins. He had actual mentors and teachers for most of those 10 years and only became on his own right before the story started.
And he didn't eek out a victory over Diana at all. He lost straight up, he even admits it himself that she's stronger than he is. He lost round 1. Manages to recover and get back in the fight become of his regen abilities, loses round 2 and gets back in the fight for the same reason, then there is a double knock out in round 3.That's two loses and 1 draw.
I don't really get why people are making a big deal out of the Diana thing though, he's not a normal Human and she's not a normal human. Mc has more potential, but she's had better training and more experience.