I started to respond to this on Friday and realized it was turning into a rant so.....
What I'll state from my rant is that if you have someone who constantly says "It's all high priority", then chances are you have some bad management going on somewhere. Maybe not your boss herself, but perhaps someone above her or in her level of managment. It can really wreak havoc on a workplace.
It may fall to you to at lease propose a prioritization if your boss won't/can't. Nothing formal, but perhaps something like: "OK Ms. Boss. What I'll do is start by calling X, and then I'll start producing the slides for the Y meeting this afternoon. I should be able to get them to you by 11 for you to review them. Then I'll work on that mailing that needs to drop ASAP. I'll see if Susie can spare a half hour or so to help me with that....."
Even if boss doesn't agree at first, or has a reason why she wants the mailing handled before the slides, you've at least given her a framework with which to start. I used to have to do this with a former boss and it forced him to hit his mental "reset" button and think rationally instead of just panicking that '"It's all urgent!"
