Svein:
To grow spirits, defeat attribute monsters to acquire XP of the dominating attribute.
Where are attribute monsters, you ask? Wind Attack Hunting Ground: Wind Plateau Earth Attack Hunting Ground: Wall of Argos Water Attack Hunting Ground: Garden of Eva Fire Attack Hunting Ground: Fore of the Gods
And the spirits you can grow at each hunting ground vary, so keep that in mind when selecting. Wind Attack Hunting Ground: Obtained Fire Attack XP Earth Attack Hunting Ground: Obtained Wind Attack XP Water Attack Hunting Ground: Obtained Earth Attack XP Fire Attack Hunting Ground: Obtained Water Attack XP