Eren had been using his soul sense amateurishly.
What he had been doing before this point was to use spiritual energy in its most basic form– to analyze the world around him in a deeper way. Or to overwhelm his opponents. As such, he hardly made use of his spiritual energy at all.
This was the first time Eren actively sought connection with his soul space. As a result, he finally found a way to employ his spiritual energy in his attacks.
Eren's fists were coated in spiritual energy when he attacked Nira Nightshade. And the attack finally managed to hit the apparition that was the Sage archer.
In the current state of his spiritual energy mastery, Eren could not use the spiritual energy in his elemental spells. But Ken's Fist Arts gave him a way to attack the Sager archer.
