An extra set of hands would help. My Silkie/BJG cross (yes, it was a weird accident) is starting to have the same issue. Boo is fine with being handled, until I start messing with those spiky tail and wing feathers - then she panics. I have someone hold her in their lap with her wings to her body. She usually hides her head into an elbow, which actually helps. Then I'm free to use both hands to crack the sheaths.
Boo's a big girl and some of the casings are really hard. I use a pair of flat-nosed pliers to gently crush those. I start near the base and work outward, the way she would when she grooms herself. I don't take the casings off, mainly because I know that's the part that hurts. It's not a problem, though. Once released, Boo gives a BIG shake, loosening the cracked casings enough that most fall off. Those that remain are usually damaged enough that she can preen them off on her own.
That's happened with her last two molts, but I'm not super concerned about a hidden issue. Other than that, Boo is a healthy, happy 7-year-old. It's just that, sometimes, we old ladies need a little help!