EDIT: it appears seraphor beat me to the punch by ~4 minutes, but i'll keep the post up anyway

personally i've mapped R3 to target-of-target, so that i can keep my party selected (via up/down on the D-pad) and quickly swap to an enemy to use my critbuff and such

you can also use macros for offensive skills that look like:
/merror
/ac "ruin" <t>
/ac "ruin" <tt>
/micon "ruin"

basically uses ruin on whatever you've got targeted, if that isn't a valid target (such as an ally) then it attempts to use ruin on whatever your target has targeted: basically allowing you to just look at your tank or party members to keep healing them, while able to fire off dps spells without wasting time changing targets (which on controller can be an absolute nightmare).
merror is there to stop the red "<tt> is not a proper target" text that pops up if you're playing solo or your target doesn't have anyone targeted themselves.

and before anyone goes poo poo macros bad, do remember that healers entire job is completely braindead, so the input limitations on macros affect healers exactly 0%