Here is the full list of all 28 Karaoke Revolution: American Idol Encore achievements worth 1,000 gamerscore.
Score 50 million points (cumulative in any mode)
Achieve Diamond Status for every song on the disc
Win the full American Idol tournament at Expert difficulty level
Win an Xbox LIVE Head-to-Head ranked match
Win an Xbox LIVE Arcade League ranked match
Win an Xbox LIVE American Idol Tournament ranked match
Play an Xbox LIVE American Idol Tournament match
Play an Xbox LIVE Arcade League match
Compete offline in a battle Head to Head game
Compete offline in an Arcade League game
Play an offline Co-op Duet
Play an Xbox LIVE Head-to-Head match
Win 20 American Idol single player tournaments
Score 10 million points (cumulative in any mode)
Complete a medley and receive Platinum Status
Achieve Gold Status for every song on the disc
Achieve Platinum Status for every song on the disc
Sing all the songs on the disc once - full song only
Achieve Diamond Status for twenty different songs
Win the full American Idol tournament at any difficulty level
Get a perfect score throughout a full American Idol tournament
Achieve Diamond Status for ten different songs
Score 1 million points (cumulative in any mode)
Achieve Diamond Status for five different songs
Achieve "Excellent" comments from Simon three times in a row during an American Idol tournament
Achieve Platinum Status for one performance
Achieve Diamond Status for one song
Pass the auditions phase in single player