In the main campaign, you can unlock raid mode stages by doing following tasks:

Stages 1 to 7: Complete Episodes 1 to 3.
Stages 8 to 12: Complete Episodes 4 to 6.
Stages 13 to 17: Complete Episodes 7 to 9.
Stages 18 to 20: Complete Episodes 10 to 12.
Bonus Stage 21: Complete the game.