There's really no good way to do it with multiple enemies. I just have a single mark and i move it when my current target is near death and it's not tied to an action.

Otherwise you might end up in situations where you mark an enemy you didn't intend to and you need to reuse the macro on the correct one or you might clear the mark if you have to do the action more than once.

Your best bet if you want multiple marks is to just have them separate on their own slots.