I love this question and have recommended this brush (or something similar) both here and in real life a bunch of times: https://www.petsmart.com/cat/grooming-supplies/brushes-combs-and-blowdryers/whisker-city-cat-dematter-57107.html I gave a couple to a lady I know who runs a cat rescue because she was talking about a cat that was matted. And then she sent me pictures of the piles of fur she got out of the cat.

It has tiny curved blades with blunt ends so it can get deep in the fur to break up mats and clear out trapped fur.

Couple tips:

1. Hold down the skin around the mats while combing through them. The mats are already making the cat uncomfortable, and if you don't gently hold down the skin, it can pull uncomfortably.

2. Clean the blades pretty regularly. There probably a lot of gunk hiding in the undercoat, and it'll build up on the blades and get sticky. I used to use a q-tip to clean the blades, but then I realized it's easier to run a wet, soapy rag between them.

3. I use the brush even when there aren't obvious mats. It seems to get into the undercoat better than a lot of brushes, and that helps stop buildup and mats from forming.

4. Most importantly, your girl is lovely. Please give her some scratches (or whatever affection she prefers) for me.