Wow, that's a really low pH. I only have experience with 3.0 pH Berliner Weisses. I think just time is all it takes, like months. How long has it been fermenting?
I don't have any batches going right now, but gravity was stable after about a week of fermentation. The process I use is a quick and easy way to make sour beers. This process involves doing all of the souring before pitching the yeast, with lacto quickly, not relying on a long slow sour fermentation, so fermentation time should
be the same as for any "normal" beer.
I've thought about fermenting like half the batch without souring, then blending to get the attenuation up, but that wouldn't work for the very sour beers.