Remember when some dope on 'X Factor' said Demi Lovato needed Auto-Tune and couldn't sing? We bet he's hanging his head in shame right now, because Lovato slayed at a Kiss FM concert without any bells and whistles to enhance her strong voice.

The 'Heart Attack' singer nailed a stripped down rendition of Rihanna's ballad 'Stay' at the May 18 Kiss FM concert in Mansfield, Mass. It was just the lovely Lovato, her lovely vibrato and a piano. She showcased her power with a moving crescendo in the bridge and final chorus.

Of course, it wasn't all heavy ballads -- Lovato knows how to have fun, too! She performed her duet 'Really Don't Care' with a certain pint-sized U.K. 'X Factor' alum. After roaring her verses and hooks, Lovato introduced her pal: "Ladies and gentlemen, please give it up for my girl, Cher Lloyd!"

The duo hopped, skipped and sassed their ways across the stage, getting the crowd to simultaneously scream and dance. We love these two together!

Watch Demi Lovato + Cher Lloyd Perform 'Really Don't Care'