The achievement guideline is self explanatory. If Expert Puzzles are giving you problems to get done, you can enter your puzzle on this site:
https://www.sudoku-solutions.com/Take your time as you do it and you can use the hints on that site and enter the numbers on the puzzle you are doing. Ensure that you do not click on the hint or helper buttons in your game or else the achievement will be voided. If you successfully complete the puzzle without clicking these buttons in your game, the achievement is yours.