I'm no macro guru by any means but, something like this maybe?
Code:
/cast [@targettarget, exists][@focus, exists][@TheTank, exists][@Player, exists] Spell
As for your PW:S macro in BGs, I believe you have two options [both of which must be handled within
ISBoxer]:

1. Replace "@Party1" with "@{SLOT1}", "@Party2" with "@{SLOT2}", etc.

2. Make an ATG and make that the 'Target" of your ISBoxer macro, then check the Round Robin box.