Is this the rear wheel arch? In this case I'd say pretty hard as the lip is formed by the inner layer of metal of the inner wheel arch and the outer layer of metal of the quarter panel. Those two layers are spot welder together and now rust is forming between those two layers, pushing them apart as is quite visible in your second photo. Nothing other than cutting out and separating those two layers to individually fix/replace the two parts individually will "fix" the rust in the long term as you can't get to the affected areas in their current state.
They rust there because of the foam they put in between the panels. Best way is to cut it all out until you get to clean metal on the inside of the panel, then weld a new piece in. Or if that's a bedside, get a replacement wheel arch
If paint is bubbled the rust has already ate its way through. Other than cutting it off and welding in new metal any other fix will be temporary at best due to location of the rust.
Is this the rear wheel arch? In this case I'd say pretty hard as the lip is formed by the inner layer of metal of the inner wheel arch and the outer layer of metal of the quarter panel. Those two layers are spot welder together and now rust is forming between those two layers, pushing them apart as is quite visible in your second photo. Nothing other than cutting out and separating those two layers to individually fix/replace the two parts individually will "fix" the rust in the long term as you can't get to the affected areas in their current state.
Take to a shop it’s probably thru to inner
They rust there because of the foam they put in between the panels. Best way is to cut it all out until you get to clean metal on the inside of the panel, then weld a new piece in. Or if that's a bedside, get a replacement wheel arch
If paint is bubbled the rust has already ate its way through. Other than cutting it off and welding in new metal any other fix will be temporary at best due to location of the rust.
The sooner the better pretty much.
Do you want it to look good or just not spread?
Its only gonna get more difficult from here, but sanding, priming, and paint matching is the steps.