Because a hero shouldn't ALWAYS win, especially if it's the first encounter. You know, like Batman kicked Bane's ass the first time that they fought, oh wait he didn't. Or when Hellboy kicked Narada's ass the first time around in Hellboy 2, oh wait he didn't. Or when Superman instantly overpowered Lex the first time in STM, oh wait he didn't. Having the hero lose or have issues the first time and then come back stronger later and finally win works because it shows a progression of their character. Also, having them win every single time kills any sort of tension, why should we take ANY villain seriously, since the hero always wins because "he's the hero." It would have worked much better if he and Sara teamed up to take out Nyssa, or if he lost but she spared his life for Sara's sake. Then when they fight later on, he wins to show his improvement. She's the freaking #2 in the LOA for goodness sake. It's good writing 101. Saying that the hero should win every single fight simply because they're the hero is ridiculous. Ultimately win the overall war, absolutely, but not every freaking fight. Sorry, that's NOT how it works.