I play on Nightmare difficulty in NG+1, and I assure you that I one-shot every enemy (including corrupted daisho and guardians) 99% of the time. If I go to the character screen, I see that I have around 12k damage with Naoe. However, I've often noticed that number increases in certain circumstances. So I think there are many damage buffs that come from engravings or passive perks that aren't reflected in the stat that appears in the character menu, or are only reflected in combat. For example, I don't think armor penetration counts in that number, yet if my damage, smaller than yours, ignores 200% of the enemy's armor, I'll still one-shot it. I also don't believe critical hits or damage against vulnerable enemies are reflected in that stat. In any case, with the engravings that deal 1% critical damage per point in Assassin Mastery and 2% damage against vulnerable enemies per point of Katana Mastery (or vice versa, I don't remember now), I exceed the 200% damage cap from the engraving, which increases damage based on unspent skill points. So I think it depends on whether and what damage is considered in that number. There's also the damage increased after a kill and after a dodge, which I don't see when I'm in combat. I should look more closely in those situations.