Here's the guide straight from Xbox achievements
The bonus level can be found in the Gotham Park of Gotham City, which is located in the center of your map. The bonus level will be shown by a large Gold Brick and will require 175 Gold Bricks in order to be opened. The objective in this bonus level is to reach one million studs by destroying everything that you can in the Lego world of Gotham. You will be allowed to use the following characters:
There are quite a few tricky spots in this bonus level which could leave you clueless for hours, but I'm going to do my best to explain to you where they are and how to complete them.
In order to get Superman you will need to reach the third part of the bonus level, which is all the way to the right of the map. There will be a telephone booth as soon as you walk across the bridge. Simply walk into the booth with Clark Kent and you will be changed into Superman.
"Purple and dark blue statues, how do I destroy them???" - This may be the trickiest part of the level, but here's how to destroy them... You're first going to want to have access to Superman. Once you have him you will need to go to the middle part of the level near the green and purple paint pumps. There will be a small platform that is sticking out of the water. What you're going to want to do is fly up there with Superman and stand on the pole. The pole will start to go down and you will be allowed to drain the paint. You will notice the Joker's Mech coming out of the green paint. That is how you will be destroying the purple and dark blue statues at the end of the level.
Once you have used the Mech to bring up the Wayne Tower, fly up there with Superman and you will notice a couple of purple studs.
When you're riding the Metro to the other side of the level you will notice a few purple studs outside. There is a terminal to the right of the Metro that you will need to use in order for you to open the doors. You can also destroy the seats inside the metro for some extra studs!
You can destroy the Church! This will provide you with a tons of coins so make sure you do it! You will have to use Superman or Clark Kent in order to take it down. You can also use Batman's suit which has explosives to blow it up.