Open the Doors
Assemble the generator pieces left off the starting point. Push the two pillars as well. Stand next to the generator and power it up once. The Scarecrow pulls the lever. Now, you are in a raised position; jump across the gap. Cut the chandelier ropes so it lands on the sculpture. Assemble the ramp and The Scarecrow will walk down to it and team up with you. Use The Scarecrow's mind control on the man in the booth to make him open the doors.
Up the Stairs
Switch to The Joker and go up the stairs and shoot the security panel to shut down the lasers. Collect the canister and keep ascending the stairs. At the top of the stairs, build the LEGO pieces into the crank and use it to power the generator and unlock the door. Flip the switch to the left of the stairs and smash the fuse box to disable the lasers. Hit the blue and white tiles and then move them to the missing squares in the checkered floor. Now that the floor is complete, you can push the box over it and against the wall. Push the metal bars into the slots and jump onto the tall pillar. Switch to Scarecrow and push the pillar right. Swap back to Joker and assemble the fan for the door to open.
Fix the Elevator
Smash the three paintings on the wall to activate the lever. Destroy the gargoyle statues and build a crank. Rotate the crank with The Scarecrow and The Joker will jump onto the platform that lowers. Switch to joker and smash the paintings to get the railing pieces. Place these onto the wall and head toward the lever. Pull the lever and assemble the generator from the LEGO pieces that appear. Use the generator to power up the elevator. Use the elevator.
Escape
Build the base board from the LEGO pieces and lean it against the barrels so you can climb the crate tower. Pull the lever, slide the box along the floor as far as it goes and turn on the generator. Jump into the toxic area as the joker and smash up the objects. Assemble the pieces to create a box. Pull the lever and the box will be lifted up. Pick up both of the boxes using the crane and place them on the two orange panels to extend the bridge. Cross the bridge with The Scarecrow and use mind control on the man in the booth so he allows you to leave.