Although I didn't have a problem with brushing, my dog is a total whimp
when it comes to other things.

3 things in particular:
I picked up some bubbles from the local pet store - the kind you blow
and dogs are supposed to chase. His ears went flat and he went and hid
behind a desk.
A drop of water would send him hiding under the table with a look of
pure torture. Any heavier water and he wouldn't come out of hiding
until I drug him by the collar.
A soft toy tossed at him would make him put on a look like I was trying
to kill him and he'd cower in a corner.

I just took everything slow. I blew some bubbles in the other room,
and when he just watched he got rewarded. I trickled water far away,
and again rewarded any sign of interest. I threw his squeeky toy many
feet away from him, however far away required for him to be unafraid.
Then slowly worked all these things closer to him with much praise and
reward. He still has problems with water, but will be around it as
long as it doesn't touch him. I admit I haven't spent as much time on
that so only blame myself. He will now stay by me for bubbles and even
pop the occasional one with his mouh. Catch has become one of his
favorite games and I can throw a hard tennis ball, soft toy, or frisbee
at him now and he gladly jumps up to grab it mid-air.

So the way I would have approached your problem, had he also been
afraid of grooming, would have been to start by just having the brush
near him. I would have left it on the ground and rewarded any interest
in it, and played with it in my hand but not paying any attention to
him. Once he was fine with that, I would have pet him with whichever
hand was holding the brush, but not letting the brush actually touch
him. Then I would have run the back of it over his body (so it doesn't
feel strange with the bristles) just a single stroke. Worked up to a
few strokes. Then turned the brush around and gone back to a single
short stroke. Meanwhile rewarding every positive reaction and backing
off if there were any negative reactions. Rewards can be treats, pets,
a toy, or lots of exciting screams (Good boy!!!! What a good boy!!!!
You're so good!!!!) depending on what your dog prefers.

This process could very well take many days or even weeks, so be
patient and don't push. Stop when she's had enough (or preferably
BEFORE she's had enough), and continue at another time.

She just has to learn that the brush isn't something scary, but
something pleasant.

Good luck.