If you create a macro that has "#ATH" in it somewhere, it will update the "/tar arena1" part to the slot that has an enemy healer.
Example macro. If you create one that reads like this:
#ATH
/tar arena1
/cast Polymorph
During the prep phase of arena, if the enemy healer is in slot 2, it will be changed to:
#ATH
/tar arena2
/cast Polymorph
Since r6 added support for focus macros:
#ATH
/focus arena3
/cast Polymorph