I don't use any /targets or /assists for targeting. I use the option target= in the commands. Also, don't ever expect your alts to target on the same microsecond your main does a command. Target on your main, then push a command for your alts. It takes your latency time for Blizzard to see who your main has targeted. Then your latency time for Blizzard to tell your alts. That is a pretty long time, so you can't do things super fast.
/cast [target=party1-target] DPSSpell
/cast [target=party1-target-target] HealSpell

You can put in stoppers if you want to limit even more.
/cast [target=party1-target, harm] (only cast if you have a target that is hurtful----calls you names, lol).
/cast [target=party1-target, nodead, exists] (only cast if you have a non Bruce Willis in Sixth Sense target, and have a target).

Hope that helps. And again, target on your main, then press the attack key (so there is time for your target to sent to the servers).