I would shave the pup if the mats are bad. You don't want her to hate grooming.
I use several brushes, but I recommend a wood pin brush and a good comb. I use the small Chris Christensen wood pin brush, but I recently bought the tiny tot brush. Wood brushes are very gentle and pull hairs away from the knot. This brush also works great around the face.
Wood Pin Brushes for Pet Grooming, Show Dogs & Cats
Right now the shipping is free on CC brushes here.
Chris Christensen - Tiny Tot Brush with Wood Pins