Back when Emrys was dealing with the Scaled Lungfish, he used his Spatial Blade ability and brought out only a portion of its power by executing the technique with a blade. To truly unleash the full potential of this ability, he needed to condense the Spatial Blade using raw Space Energy instead of relying on a weapon.
This time, he could have condensed the Spatial Blade to easily deal with the giant serpent, but he still felt that this beast was not worthy of him using this Origin Skill directly. Instead, he decided to manipulate spatial energy and use the spear in his hand as a blade to execute the Spatial Blade.
Since he was using a different weapon, the strength and sharpness behind the attack might decrease significantly, but it would certainly increase the lethality of the attack—perhaps enough to kill the serpent.