Unlock Artifact Temple Stage For Single-Player
Just get to the Artifact Temple in Multi-Mission Mode. Once there, the stage should be rendered unlocked when your session is over. You will need twelve artifacts to reach the stage initially.
Unlockable Expert Difficulty
In multi mission mode, defeat the Metroid Prime on the Impact Crater map.
Unlockable Impact Crater
Clear each board destination in multi-misson mode.
Unlockable: Phazon Mines in Single Mission mode.
Defeat the Omega Pirate in Multi Mission mode.
Unlockable: Phendrana Drifts in Single Mission mode.
Defeat Thardus in Multi Mission mode.