Arts is reduced by a % while Phys is reduced by the raw defense number.
Even facing a unit with 70% res, arts will still do damage. If Exusiai faces a unit with 700 defense, she will have her damage reduced to basically nothing. For example, the Frost Drone requires a full unload of Exusiai's S3M3 to take down, but requires much less effort to kill with Casters despite it sporting a B Def rating and a B Res rating. There are also a couple Arts Guards that deal Arts melee damage. These are Mousse and Astesia.
As for Ceobe I ignored her banner so idk what she does, but AoE casters all share the general characteristics of having enormous DP cost and shorter range in exchange for dealing AoE damage. I don't use them a whole lot but they're generally meant to deal with either enormous crowds of low res enemies or small clusters of high Def enemies like heavy defenders.
Exusiai does do a lot more work on a lot of maps though, because she has extremely fast attacks.