From LEGO Batman Wiki
Destination Metropolis is the 8th mission in LEGO Batman 2: DC Super Heroes featuring Batman and Superman. It starts with Batman flying Robin's Helicopter and Superman, just flying. They are following The Joker and Lex Luthor through the skys above Gotham City,
First you must destroy the turrets and helicopters around the Airship and then destroy the engines. Batman will then shoot his grapple hook at the cargo door, opening it, and then he and Superman will jump in.
Once in the aircraft, you will first need to destroy the LEGO bricks which are controlling the shield. Once destroyed, you can freely enter the ship. Find Batman's Electronic Suit which you will use to power the elevator. Then, use the electronic suit to make Batman walk past the green light and then cover it with the buildable LEGO blocks. The next room is where the second to last fight happens. Enemies will keep respawning in the spawning doorways which you will need to cover. To cover them, steal one of the jetpacks and shoot its rockets at the shiny silver boxes containing the buildable LEGO blocks which you will need to cover the doorways. Destroy them and build them onto the doors. The other boxes contain mechanical arms which, once built, will pick up a bomb from beneath the aircraft. Use Superman's Heatray to light the bomb then watch it blast through the door.
Run through the destroyed door and fight the enemies. Find the Bat Suit and use it to break both the glass blocks and infiltrate the cockpit. A cutscene will follow where Joker opens a trapdoor making Batman fall out. Superman continues floating until he notices that Batman has actually fallen out. Superman flys out, allowing the aircraft to escape, whilst he saves Batman. After this, the mission will complete and you will unlock Superman as a playable character for Freeplay.