I agree with Almeeker. I would sit down with the manager and have a heart to heart. Just keep all emotions out of the conversation and explain that your priority is to have things running efficiently, not letting drains get clogged, or vaccuums overloaded and that this assistant seems to have made not taking "orders" from you her priority. She clearly has a problem with authority and at her stage in her career that is just going to consume her work life with conflict.
You can even rationally explain that to the assistant herself. Again, I would use every ounce of restraint I had to remain emotionless and just tell her that you have no desire to "boss" her around, you just want things to run efficiently and you have the benefit of 8 years of experience behind these suggestions. Also next time she starts to argue with you I would just say, "I'm not here to argue with you. I am just doing my job." and walk away. Don't engage her. She sounds like a lazy, spoiled, child and her problems will follow her to every job she gets. You on the other hand, are an experienced professional with a great head on your shoulders so don't give the runt any power over you. Stick to your guns! The critters need you, Lizzycritter!
I hope your venting helped!