Stan Hulk is automatically available upon completing the final (assuming you've already found Stan in all mission and side-mission) "Stan Lee in Peril" location, on board the Helicarrier, in Deadpool's room. After you've done so, open the character selection screen (for me he was actually automatically placed as my main character) and select "Stan Lee". Hold down Y and watch as he transforms into the mighty Stan Hulk! The achievement will pop immediately upon transformation.
Stan Hulk is also one of 5 total "Big-Transformation" characters needed to unlock the "Alter Ego" Achievement.
For a list of all "Stan Lee in Peril" locations/ solutions during missions and throughout Manhattan in Free Play, please refer to my comprehensive guide on the "Stan-tastic" achievement page!
- If you found this guide to be helpful, please vote and send me feedback! Be sure to check out my many guides and solutions for achievements in Lego Marvel Superheroes, and I'll help you towards achieving that 100% completion as I have. Thanks!