The functionality is already in game, sorta..... Focus Target
Use macros to set up your friendly target skills to cast onto whomever you have your Focus assigned too.
This, then, requires no target changing.

To use a controller to mark your focus target:
>Target
>Press Square (opens tiny menu)
>Up on the d-pad (instantly scrolls to bottom of tiny menu)
>Press X
>Target focused.


Make a macro for the skill you will be using on the focus target and assign it's icon to your button bar.
/ac "Name of the Skill" <f>

Example:
/ac "Clemency" <f>