If I, for my sins or by good luck, run into some giant out there, as usually happens to knights-errant, and knock him down in one charge, or split him down the middle, or in short beat him and make him surrender,

Giants are the standard enemy of the romances, and Don Quixote treats meeting one as a matter of course, de ordinario, a thing that happens to men in his trade. He cannot decide whether it would be bad luck or good: por malos de mis pecados ("for my sins") and por mi buena suerte stand side by side. In chapter 8 he will find his giants, and they will be windmills. All the fighting is in the present tense, derribo, parto, venzo, rindo, as if it were already happening. There is a small echo too: me encuentro, "I meet", and then de un encuentro, "in one charge". The meeting and the clash are the same word.