Wood element

Yes, controlling the elements was the crux of the problem.

A person who cannot control the element can never be a rank 2 warrior. 

Even if Aru had unlimited spirit energy and the fitness needed, without trying to control the elements, he could never rank up.

He was met with another roadblock. 

But that too was solvable.

'I just need to use my spirit energy and give it the property of the preferred element, right?'

Aru's body was brimming with energy. All the muscle tears had healed and he was in pristine condition.

'First, I have to gather spirit energy, channel it focus it and extrude it out of my fingers.' While thinking, he began to manipulate the spirit energy.

And as he wished, a small blob of energy appeared on his fingertip.

'Sucess. Now I have to turn it into some kind of elemental reaction. Should I call it elemental transformation, hahaha.'