/cast [target=target, exists, nodead, harm] Fireball
/stopmacro [target=target, exists, nodead, harm]
/cast [target=focustarget, exists, nodead, harm] Fireball
/stopmacro [target=focustarget, exists, nodead, harm]
/cast [target=pettarget, exists, nodead, harm] Fireball

That's off top of my head, and untested.