@PerfectDeath, it's definitely luck that he actually given chance to somehow use his "Cutting-attribute" weapon effectively on the boss when chapters before he already stated that his weapon is ineffective on it thanks to its armor.
Also, i like the fact that the fight is still technically a chivalrous one because the Boss is the one who used weapon first, making it that the MC's usage of weapon for the finishing blow himself fair.
Heck, the moment where the boss underestimate the MC by trying to use his sword instead of his barehands to finish the MC could very well be the "Lucky Break" that allows for the MC's victory.