Celine didn't bother fighting Nial after he began to wield the spear. She knew that there was no way for her to overwhelm him. Not only was her combat experience insignificant in comparison to Nial's experience, but her physical strength was lower as well.
There was not even the need to mention Nial's One with the Spear state or the fact that Nial could undergo a partial Soul Fusion, the process in which the Familiar enters the Summoner's Mana Core to fuse their bodies to a certain extent, allowing the Summoner to manifest his Familiar's body parts on his body.
This was something that could only be done if both the Summoner and Familiar trusted each other fully and after years of tempering their firm connection.
Most would never be able to create a tight enough Soul Link to initiate a partial Soul Fusion. That was not because they treated their Familiars badly but because the Soul Link had to be extremely firm.
