What I do for my priest (since I use a UI frame that enables a slot to show a focus target, is this macro:
/focus [modifier:Shift]
/cast [target=focus,harm] Shackle Undead; [harm] Shackle Undead
/stopmacro [target=focus,noharm]
/clearfocus [modifier:alt]
I hit Shift and click on it to focus the target, then whenever I want to clear it alt and click on it. Otherwise, spam the macro every 15seconds or so.
But, if you want a mod, I cannot seem to find a good one lately outside of using DoTimer or ClassTimers.