Wild Jango: Defeat him in the Central Tower Chapter 2.
Silver Horn: Defeat him in Tianna Camp Chapter 3.
Mad Nautilus: Defeat him in Gaudile's Lab Chapter 4.
Mach Jentra: Defeat him in Ulfat Factory Chapter 5.
Incentas: Defeat him in Gimiala Mine Chapter 6.
Depth Dragoon: Defeat him in Far East HQ Chapter 10.
Raflesian: Defeat her in Tianna camp by using Mechaniloids to get the Tianna key.
Ninetails: Defeat him in Central Tower once you have the Central Key. Note: To get the Central Key you must complete the game.