curbstomp humiliation battlespar between Jiro and Kazuma), though I tend to take it a little more easily on my mortal world characters. That said, we've rigged battles to go one way or the other for the sake of the story before.
I've also made it a point to give each of my characters at least one major flaw when it comes to combat - Kazuma, for example, is exceptionally skilled and strong for his age as a shinigami, but he's also incredibly reckless; Hasunaga pretty much has a time limit on how long he can fight (given the way his Reiatsu generation works), plus he has to fight in a fairly specific way, and he doesn't do well on continuous battles; Reiji doesn't have the means to move incredibly fast like other characters can, and has no means to survive violently massive wounds (unless we're going with this); Rouga is slow and, despite having massive defensive power, has fewer attack capabilities to speak of; Serazel has little attack power, so he's screwed against an opponent with high defensive power (plus psychological issues); Subject 1864 is limited to how much power he's allowed to let out at any given time, which is often probably not enough; Shishiou can't make use of the full extent of his shikai on all points at once, so if he misjudges which one to use, he gets hurt badly (plus issues when he goes into hot blood mode); and finally, Soushi has limited defensive capabilities and doesn't stand up that well to pressure in melee.
Fundamentally, I believe there is no perfect fighter. Each and every last one has at least one major or minor flaw that can be exploited to achieve victory in some way, and even being aware of that flaw does not necessarily make it go away; worse, if it makes the fighter worried, he'll be unable to focus on other things. Some fighting styles also have inherent strengths over others, which makes some match-ups more likely to go in a certain way than others (though it's by no means predetermined). These are only two in a list of endless things that we can use in narrative to make a battle end satisfactorily before we resort to something like "luck".