I don't /target mobs, ever. I just assist my main using focus:
/cast [target=focus-target] DPSSpell

But for checking for mobs, try:
/cast [target=Astral Spark, nodead] [target=Curator] DPSSpell
dead, harm, help automatically check for exists, so you don't need both

If you have to use /target, then try:
/target [target=Astral Spark, nodead] [target=Curator]

Hope that helps. Good luck.