The problem is how to apply unscrewing torque without applying extra forces, sich as side squeeze, which increase the friction and make unscrewing harder. Get a really grippy flat mat such as a silicone or rubber sheet. Place on a hard flat surface such as a table top. Push the hood adapter firmly vertically down on it,and then twist. This applies the maximum unscrewing force along with the minimum side forces which would increase the friction and can distort the shape, increasing the friction even more.