I've been there, trust me!
I simply spray cool water, perhaps with a little dish soap or insect soap, up under the leaves. Twice per day at first then once per day and then just every few days. The mites cannot handle high humidity and will leave or be dead after a few days. Very briefly raising the humidity on a regular basis like this can be a good way of battling many different bugs as they tend to like it either dry or humid, but not both. Since I have done this I have had no real bug issues. By the way, once in a while add a few drops of neem oil to the spray if you have it, then you have a really solid defense, but don't use neem in late flower.

This works, every time, it's work at first but soooooo worth it.