Rare Replay

Play the game one time trough. Afterwards only the secret Sky Stage will be missed. To enther this Stage do this:
Go to Arcade Mode, press Start on your second Controller to enter the charakter screen with your second player too. Now both player choose their fighters by holding DOWN and pressing MK (=Medium Kick).
This video shows in the beginning where you have to input the combation: At 00:02 you can see written: "Sky Stage"
If you don´t want to play the game trough choose the stages manually this way:
QP = Quick Punch, QK = Quick Kick, MK = Medium Kick, MP = Medium Punch, FK = Fierce Kick, FP = Fierce Punch
Up+QP Wolf Castle
Down+QP Bridge
Up+QK Stonehenge
Down+QK Dojo
Up+MP Jungle
Down+MP Dungeon
Up+MK Museum
Down+MK Spinal Ship
Up+FP Space Ship
Down+FP Street
Up+FK Helipad
Down+MK Sky Stage (both players must do this)

If you use the default button layout, the combinations for the stages are: (Credit to -> "Berk Porter")
Wolf Castle: +
Bridge: +
Stonehenge: +
Dojo: +
Jungle: +
Dungeon: +
Museum: +
Spinal Ship: +
Space Ship: + or +
Street: + or +
Helipad: + or +
Sky Stage: + (must be done on two controllers at once)

Hint: This achievement can also be done in Practice Mode
Credit to-> "Fleshwrither"