Because of those ages, characters in the first category pretty much can only beat characters in the second category through such immense willpower or literally beyond superhuman fitness. There's just no possible way any Mortal could amass a skill in combat greater than that of a Shinigami, because the Shinigami are older (barring exceptionally young Shinigami or, like, Bount). So their immense willpower would have to cover that weakness. And even then they'd need fitness to match someone who's trained for combat situations for (at least for most Shinigami) half a century at worst. Seems unlikely.
Luck is always going to be a huge factor in any of the threats the Mortal cast faces. It will always be some combination of Willpower ("I have to win" vs. "They're just kids") and just dumb Luck.