Such a unique and iconic line!! You sitstart on the backside of the arch and dive underneath using holds as if they were made just for this purpose. Of course, the finish involves a fine mantle ;)
Rain sucks, but broken holds suck even more. Remember, never climb on wet rock in Fontainebleau.
The sandstone becomes brittle, and pulling on it will ruin classic problems. Drying it doesn't help, because it's still wet and weak inside.
All you can do is wait for better conditions.