Painwheel's main issue is her poor air-priority and the lengthy start up coupled with limited range of her safe normal's. As a result, neutral's rough for her. Seems that people use Cremation (C.hp) or Ashes to Ashes (F.hp). Both are long multi-hitting attacks so they keep an opponent busy so PW can re-position or try for risky mix-up. I think Cremation will give you a much needed chunk of space control that you can confirm into damage, while Ashes to Ashes will take away range in exchange for a slow overhead for some potential mix-up with easy combo-ability...
If I had to pick between the two I'd go with Cremation. It covers a big chunk of space, so you can make jump or flight mix-ups safe...er
Ashes to Ashes, once it's seen, is too easy to block, especially when you consider how Skullgirls handles "Unblockable mix-up".