at-root + modifier

*.scss

@media screen and (max-width: 320px) {
  .Notifier {
    $block: &;
    display: block;
    
    &_Main {
      color: green;

      @at-root #{$block}.-error #{&}{
        color: red;
      }
    }
  }
}

out.css

@media screen and (max-width: 320px) {
  .Notifier {
    display: block;
  }
  .Notifier_Main {
    color: green;
  }
  .Notifier.-error .Notifier_Main {
    color: red;
  }
}