That was the last achievement that I've been needed... The fastest way I got this: Go to "Frozen - Let it go" --> start the "Elimination Mode" (if you've not you must unlock this by sing five times in a row perfectly on Expert Mode or spend 5 VoxPoints <-- I hope that's correct), you need two mics. Not sing now. After 14 seconds, the song ends and counts for the Achievement. Start the "Elimination Mode" over and over again until the Achievement is yours.