Q:

How do I deal with a coworker that keeps changing the look of a report I'm building, without documenting the changes in a design template?

Ideally this was to be an easy project.  Create a report for a website. Something like this:

I am provided a template (drawn up by graphics designer)
I implement/update the programming to match (codify)
Repeat 1 or 2 as needed if I miss something or more changes/updates are needed.
The end.

Instead, it appears this coworker lacks the required workflow on how to go about this project, and abuses my time to visualize output of her changes to the project.  Namely, she is not utilizing the graphics department to draft the proper template.  She makes updates to the template herself, often on the fly, while requesting my input on those changes.  Usually ignores my input. 
Typically this goes as such - coworker is either repeatedly stopping by my desk at random intervals or requests me for spontaneous meetings, and tells me things like these

omg, I am "so done" with this project
can you make this change today? today right? Today(?)
can you make this title smaller?
can you make these cells the same width?
can you make this text bigger, right?
does this (font, title, margin, space, logo) look right to you? I don't know what do you think? (repeat 3-4 times)
You're going to do this today, right?
can you make this wider/smaller/bigger/shift left/shift right
what do you think? 

Repeat all of the above the next day. There is no template even though my boss asked her to provide one.  The template she did provide was trampled over by her (back to the change process described above).
I am not a graphics designer. I can match the report to a given template, but I don't care, nor should I provide guidance on what the report should look like.  These constant changes and spontaneous meetings can last 30-40 minutes, where coworker repeats her changes, mulls over them indecisively, for the duration of the event, while I am getting ready to shoot myself figuratively.
One meeting in particular, an interaction triggered an anxiety attack. I had to use controlled breathing coping technique to keep myself calm. This should not be a typical workplace response. 
The interaction was when I reluctantly kept agreeing to implement incoming changes, which only produced more change requests, and she still was not sure if those were the changes she wanted to be implemented.
In reality though I am also done with this project, even before she starts her next change avalanche.  She is keeping me from advancing to other more important projects.  (Projects that do not involve making fonts just a tad smaller/bigger/wider/narrower ad infinitum)

A:

I think the crux of the situation is:

There is no template even though my boss asked her to provide one.

Next time she stops by, beat her to the punch and insist that you need the template to do your job before she even asks for the next iteration of request.  When she derails the conversation with specifics of what she wants wholeheartedly agree that that should be a part of the template that she will provide you.  Do not agree to provide this report again without a template unless you get specific instructions from your boss to do so.  If you are feeling it, drop by her office unannounced and ask for the template that she was supposed to provide you.
As your boss is aware but not taking action, let them know that you will not be fulfilling future requests from this person unless proper business processes are followed.

A:

When she talks to me my anxiety goes through the roof. I have to use
  controlled breathing while imagining murdering her in a variety of
  ways. This is bad.

You should be discussing this part with a mental health professional. 
They can advise strategies and/or medication to help you cope with your anxiety, since you find it so severe.

What do I do exactly because even in my infinite wisdom I kept a poker
  face telling her grits teeth "okay I will implement those changes" In
  reality though I am also done with this project, even before she
  started. She is keeping me from advancing to other more important
  projects.

You should be discussing this part with your boss. 
Together you should decide how far you should be going to please this coworker (and any other consumer of your work). And together you should decide how to deal with them when they want more than you are authorized by your boss to give.

A:

She is keeping me from advancing to other more important projects.

That should be important to the organization's leadership. So, you will hopefully have your manager's support when you set a firm deadline beyond which further edits cannot be made.
Here's an example. "I am unable to make any additional changes to this document after [insert the day of your choice, including today]. Please make sure to give me a complete list of all the final edits you'd like before then."
If they have a change after that deadline, take it to your manager and let them know how and why the situation is preventing you from addressing more important projects.

