Before you start your season/franchise you should edit rosters and trade all the top players (85+ rated) onto the team you intend to play with. Start the season and pick your stacked team. You should be able to sim the entire season and make the playoffs. Save the game and sim through the playoffs. After each series you win, save again. If you lose a series reload your last save and re-sim it. Once you win all 4 series in the playoffs you will win the stanley cup. The achievement will then unlock.