Relying on devouring another's divine soul to break through is extremely difficult. Moreover, it requires ensuring that the target's divine soul is sufficiently powerful—specifically, one that has been stripped away from a mighty expert.
According to Xiao Yang's estimation, Qing Honghai's original strength was at the Nascent Soul (Great Completion) stage, at the very least.
Furthermore, devouring divine souls has massive drawbacks; it runs a high risk of splitting one's own self into two, even if the opponent's memories are successfully wiped out.
Chapter 388