MechaPilot
Explorer
If you're looking for an attack description in terms of fiction : I get inside the guard and shoulder shove it to gain time to attack the side stripes and buckles holding the breastplate together. Its arms having gone up and out a bit from the shove, the right (their left) side will be exposed.
After an initial downward swing on the straps/buckles, I can probably reverse grip and use the counter twist to "grab" the edged of the breastplate with the sword's guard which will give me great leverage to really bend those edges or get in some excellent sawing at the remaining straps and buckles.
... or something.
If a more "general" idea is required : "I hit the shoulder-pads with a exterior-going swing" or "I time my attack to strike from the side and then stab going in through the side but angled so I won't really hurt the foe - but the armour will get a decent rent in it."
... or something else? I don't know.
It's sort of amusing to me that your explanations are actually more detail-oriented and less abstract than the AC system the game uses. As a DM, I would really only hold a player to the same standard that the AC system uses.