Has anyone ever had significant, ongoing issues with a co-worker?

Some back story might help... I started at my job in February. I have an "assistant" whose worked here for 5 years or so. She's about 15-20 years older than me. Single until recently, I think (this is relevant, promise). I've had issues with her from about a month in and recently it's gotten worse. They affect my productivity, ability to handle/manage my cases and my well-being. At this point, something needs to change but I don't know what or how to go about it.

She cannot and does not communicate at all. When I ask for things to get done, she doesn't let me know if she's busy or whatnot and I'm stuck constantly "following" up to remind her. It's annoying. I give her deadlines and she chooses to look at them as guidelines. They're not. She takes it upon herself to make decisions concerning my work that are not her decisions to make. Her attitude is the worst I've ever encountered. Ever. She's unprofessional, rude, confrontational, condescending... I hate having to approach her with things because I never know what sort of fight I'll get from her.

I've tried playing her game but 1. that's just not me and 2. it's annoying and tiring. I don't know what to do at this point. I'm suppose to sit down with my boss tomorrow. We met as a "team" a month ago after I talked with my boss about the problems I was having and my assistant hasn't changed a thing. I'm at a loss for what to do.