This is the first time he is using soul force to fight directly. Origin gods use it to push and pull the law matrix. It is a more powerful means of the manipulation of laws but rarely do Origin gods use it to attack each other. Only immortals with spiritual concepts use soul force directly but those are rare.
Unfortunately for Legion-7, there is no law matrix or laws in this place so he can't use soul force for what he is most good at. The best he could use it for is to defend himself. However, that turned out to be enough.
This is because the soul fragment was being ground down by the grey corrosive water around them. The two of them are not the only ones in this fight. The third combatant is everywhere and is attacking everyone indiscriminately.