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"