A Navi Stage was dependent on two things primarily.
The physical capacity of a person to collect the Navi and the spiritual capacity of the person to use the Navi. The body and the mind together needed to improve in order for the person to move up in the Navi Stage.
It was very commonly believed that the process of going up a Navi Stage was what led to a person's Navi quality improving. However, the truth was that improving one's Navi quality was the defining factor that moved them up a Navi Stage.
And the quality of the Navi depended entirely on the person's body and mind.
To improve both, all one had to do was continue gathering Navi and using it through their body and mind.
In the case of a Warrior, who did not have a Sea of Navi, they gathered Navi into their body and made use of it constantly. That very reason was enough for a Warrior to have constantly improved their body with their Navi.