All-Brett beers act similar to regular beers. 70 to 75% attenuation is normal.
Did you mash high (lots of unfermetables), or use a lot of specialty grains. Just like regular beers, they will leave a sweet beer.
All-Brett beers are not like regular sours, and the yeast is not super-attenuative.
Someone mentioned racking to give some O2. You can give it a try, but I don't think it helps. I do it all the time (to harvest the yeast after a couple of weeks), and it doesn't seem to have moved it.
What results have I had? My latest is at 1.015 from 1.060. I've had a couple similar, and I had one that went from 1.058 to 1.008. All same yeast (WLP650, Brett-B) from the same parent vial.
If you think you have enough fermentables, you can try adding an active starter of another yeast to get it lower.