Actually, disarming is notably more difficult than fighting defensively or taunting. Riskier, too. Comparitively speaking, maneuvering one opponent into the way of another opponent is much easier (at least for awhile). Which is another reason why this ...
... is a good idea. Because what is "easy" or "hard" is going to change by genre and the sensibilities of those at the table. If you are playing a Three Musketeer type game, disarming should be easier than it is in any kind of more real-world system. And naturally that is even more true in some more fantastical settings.
This is also why I disagree with the hard distinction between magical and mundane, here. Some of the things that people want as mundane are magical--precisely magical in the sense of "made up, fantastical things," like using a double-bladed sword effectively or rapidly reloading a crossbow.If you have stances, you can easily label them not only for difficulty (anyone, expert, master, etc.) but also for the how fantastical they are.
I'd go for untrained, trained and mastered. Three steps.
Half the stuff we assume all warriors to be able to do in D&D is rather difficult. Then we have to remember that only so many enemies are human sized humanoids. So id expect most untrained moves and stances performed just by imagination and watching other would be right rolls of 17 or better. Equivalent to button mashing in fighting games. Maybe advanced button mashing.
D&D's mundane is near ridiculous. Most rpg mundanes are crazy.