Complete these levels once to unlock multiplayer stages:

Planet Sauria: Beat Sauria: Reunion
Fichina: Beat Fichina: Into the Storm
Outer Sargasso Hideout: Beat Sargasso Space Zone: Hostilities Revisited
Katina Outpost: Beat Katina: Frontier Base Battle
Inner Sargasso Hideout: Beat Sargasso Space Zone: Hostilities Revisited
Space Station: Beat Orbital Gate: Incoming
Corneria City: Beat Corneria: War Comes Home
Aparoid City: Beat Aparoid Homeworld: Breaching the Defenses