"Heroes of Light and Shadow" features bonus content that can only be unlocked by connecting online. If you have a flashcart and a Wi-Fi connection, you should be sorted.

If you're playing on emulator, you can use the save files provided here to access the bonus content. The method of applying the save files varies according to the emulator, but is generally the same.

Firstly, rename the save file so it matches the ROM. In most cases, it should be "Fire Emblem 12-patched".

NO$GBA - Place the save file in the BATTERY folder.

DeSmuME -  Place the save file in the same folder as the ROM. If in doubt, try importing "Backup Memory".

WARNING: save files can NOT be combined. So if you already have a save file, using one of the save files means you must start right back at the beginning.