Any tips on a good way to approach this (or mentally prep myself)?
The main way to prep yourself is to have the ultimate belief in the fact that you did not deserve her tirade and she had no right to speak to you that way. Also, keep in mind that if you don't say anything, you'll most certainly receive more of the same from her in the future. You may be on the receiving end of her griping again anyway, but I can almost guarantee you that it will be less intense because you have stood up to her.
As far as how to approach her, you don't have to be angry. Be cordial and pleasant, but firm. Make sure that she has 5-10 minutes to talk (turn off the phone ringers or go into an empty conference room if you have one) Keep a good, strong eye contact with her. State that you were very bothered by the situation that occurred and you want to make her aware of your thoughts on the matter before more time passes.
Speak slowly. Take deep breaths. And, most of all, don't let her take the conversation off track.
For example, if she says, "But the files needed copying! You've let them sit there forever! Do you think they'll get done on their own?"
You reply, "Nevertheless, it is not your place to tell me how and when to do my work."
If she says, "And, I've just not been satisfied with how long it's taking you to complete that file you've been working on. It should have been done last week!"
You reply, "Nevertheless, it is not your place to tell me how and when to do my work."
Repeat as necessary.
When you've said your piece, end the conversation so she doesn't have a chance to keep re-hashing it.
Just say, "This conversation is done. I hope you'll think over what I've said. I really want to have a good working relationship with you."
If you find that she absolutely refuses to accept the veracity of what you are saying, you need to state that is appears that it is necessary for the boss to become involved. Then, go set up the meeting.
