How to destroy granite black rock

I'm going to have to issue a mea culpa here. When I answered your previous question, I found a lot of people who watched the video and then were stuck in the potions room, but there were also a few that made it sound like they were able to continue playing the game. I assumed that the people who were stuck had not properly activated the pictures that open the exit, so I advised you to watch the video to solve your problem.

I have now spent even more time going through a bunch of gaming forums, only to discover that the huge majority of people are actually stuck in the potions room after watching the video, with no way out. The few that were able to continue seemed to be people who had discovered the Polyjuice Potion problem before finishing the game - likely before the Potions classroom changes to the one with the Aging potion in it.

To make a long story short, I'm afraid you are stuck in the Potions classroom and unable to complete any of the Hogwarts part of the game without starting a new game.

Unfortunately, there are bugs with the co-op mode in LEGO Harry Potter: Years 1-4, which can result in any of the "buy all types of characters" trophies not rewarding properly. I hate to say this, but the only solution is to restart the game from a fresh save and ensure only one player purchases the characters.

From PS3Trophies.org:

Q: Help! My Wonderful Weasly/Quidditch Team/Dark Wizards/Animagus Trophy didnt unlock! Why? What can I do?

A: These trophies are known to glitch if any of the characters needed for the trophy are purchased by the player 2 in co-op mode. There is no known remedy for the situation so far aside from starting the game over for that trophy. The best way to ensure these trophies unlock normally is to make sure player 1 purchases them all. Sometimes it is even better to be safe and have player 2 (if playing Co-op) drop out while you purchase characters.