In order to unlock this achievement, you have to complete a puzzle where the last seven numbers solved are 7's.
In order to do this, you will need to have a puzzle that have at most two 7's displayed at the beginning of the puzzle (in black).
You actually don't have to leave all the 7's until the very end. You can enter 7's into the correct squares, just make sure you don't solve the puzzle. Once you get to the final number before solving the puzzle, erase the 7's that you entered/solved yourself (in blue) and then enter that last number. If you now have just seven (or more) squares left blank that are all for 7's, you can enter them in last and the achievement will unlock.