AOE Specials?

It does damage based on the type of damage the user of the special already does. If an axe unit uses it, it's based on foe's def; if a red mage uses it, it's based on foe's res.

I doesn't say that it deals damage using the enemies lowest stat, it's based off of what stat you are targeting aka physical vs Def, Magic vs Res

That will only happen if you use a dragon or Felicia (or the new BS skill that's gonna powercreep them hard).

Only Felicia can target the lower defensive stat thanks to her weapon.
Dragon can do that too but they need to attack a ranged unit.