Compared to the numerous odd tricks up the Mages' sleeves, the means of fighting available to the knights of this world were way too primitive and crude.
Even though the spellbreaker knights and the Mages were of the same grade, and weren't too different in terms of their level of strength, they had no methods of restricting and containing the Mages other than a couple of their battle techniques such as their Flame Blades, Explosive Cross Slashes, and Crescent Moon Slashes.
In comparison, the Mages had confusingly strange spells and unthinkable means of slaughter. Together, these factors allowed the Mages to easily kill a spellbreaker knight with a single wave of attacks.
The difference in power between beings of a higher plane and beings of a lower plane was immensely huge, even when compared with the same grade!