Instructions
Build a generator using the parts on the floor near the steps. The generator will not yet be completely functional. There are two pillars at the top of the steps; push them along the checkered floors to reveal a lever. Use the Scarecrow to pull the lever, raising the generator halfway. Now, use the Joker's electricity attack to raise the generator even further.
Jump from the generator to the tops of the pillars and move along them to the right. As you move around the upper floor, you'll see the controls that hold the chandelier up. Smash them both, causing the chandelier to fall to the floor.
Use the smashed chandelier pieces to build a wooden ramp leading up to the double doors. Climb the ramp and have the Scarecrow convince the guard inside the booth to let you through the doors. Proceed through the doors, into the gallery and up the stairs.
Have the Joker shoot at the security panel behind the lasers to deactivate them. Use the LEGO pieces you find to assemble a crank and rotate it, then activate the generator using the Joker's electricity and head through the doors into the next room.
Avoid the cloud of poison gas with the Scarecrow. The Joker is immune to it, so move him through the cloud and up the stairs. Keep heading to the right and up, and you'll find pieces with which to build a fan. This will blow away some of the poison gas, but not all.
Walk back to the stairs and head to the left. Hit the switch you find there to reveal the panel, then break the panel to turn off the lasers. Behind the lasers are blue and white tiles. Bash them to loosen them, then use them to fill the gaps in the tiled floor. Once you've repaired the floor, you can move the large block and jump on it to get to the next floor.
Assemble the metal railing and have one of your characters jump first to it and then to the top of the tall pillar. Switch to the other character and push the pillar along the tiles to the right. Jump to the next platform, find the fan and assemble it. With both fans working, the gas will blow away and the door will open.
Enter the next area and smash all the paintings along the right wall, revealing a lever. Smash the statues and use the pieces to build a crank, then use the Scarecrow to lower the elevator platform while the Joker hops on. Assemble the metal bars, then use them to jump to the area where the lever is. Use the lever, which will cause blocks to emerge in the lower area. Assemble these into a generator, then shock it to restore power to the elevator and leave the area.
Smash items in the next area and assemble a pallet from the parts. Use the pallet to jump over the fence. Once you're over the fence, pull the lever on the right. A box will emerge from the machine. Push it to the conveyor, then shock the generator so that the conveyor moves the box to the other side of the room.
Jump into the toxic waste-filled area as the Joker and break up the objects you find there. Use the pieces to assemble another box, then pull the lever to lift the box out of the waste and onto the floor of the far side of the room. Enter the orange control area and use the crane to place the boxes on the orange panels. A bridge will now extend across the waste. Cross it as Scarecrow and persuade the guard in the booth to let you out.