Flying Demon used his individuality to stop the creature for a few instants, and Dreaming Demon exploited that window to affect its mind and prolong its defenseless condition.
The hybrid's sea of consciousness was only in the fourth rank, so the Demons didn't have any trouble in affecting its thoughts. Dreaming Demon could render their opponent defenseless and unleash attacks without risking their lives.
Destroying heads wasn't easy for them. Their attacks bordered the beginning of the sixth rank, but they lacked Noah's might. Yet, Dreaming Demon's mental waves affected the fabric of their tissues and made them frailer.
Their battle had a slow pace and saw a constant cycle of attacks and retreats, which depended on when the Hydra recovered its senses. They weren't as flashy as Noah, but they didn't suffer any injury even after four heads had fallen.