Spirits
The Spirits are classified into 4 Levels of Hierarchy, those are:-
The Low Level Spirits, referred as Chloros
The Intermediate Level Spirits, referred as Aetherias
The High Level Spirits, referred as Astrafylix
And Finally The Spirit Rulers of the Spirit Realm.
If you can form contracts with either Intermediate or High Level Spirits, It can be incredibly advantageous, especially when facing multiple enemies and you have to manage mana efficiently while casting multiple spells simultaneously.
It was easier said than done though, just because It is easier to form contract with them doesn't mean you can form contract with them anytime, they are extremely prideful and prudent creatures, they won't form contract unless you are strong and will prove useful for them in the future.
(The bond strengthens as both the caster and the spirit grow stronger through the Mana Contract.)
You need to demonstrate not only your strength but also your commitment to continuous growth. If a Spirit supports you and sign a Mana Contract with you, they expect you to uphold that promise, as the stronger you become, the stronger they will grow in connection with you.