Load up any puzzle and make sure that there are at least 7 7s that need to be filled in. I used expert to have more numbers available to fill in, but you could get this on any difficulty. I turned on all the helpers on the left hand side and went through and filled in all of one number at a time, leaving the 7s for last. Will unlock immediately.
*edit* It's worth mentioning that if you have less than 7 7's you can just keep placing them and removing them and placing them again until you've done it 7 times. from T Svectra T