Once the battle started a lot of spells were launched from Evior's side. Almost all the students on Evior's side fired random spells towards Kretos. On the other hand, the guards who were paid to protect Evior didn't fire their spells as they tried observing what Kretos would do facing a bunch of low-tier to mid-tier spells.
Unfortunately, the students had no battle experience, nor did they have good cooperation skills. Some of them canceled one another spells as they collided mid-air. Still, even though some of the spells didn't make it to Kretos, a majority of them were able to reach him.
Kretos used the same spell as the Elder Lich he fought before. This one was a modified version based on his understanding. he still doesn't remember anything, he just knows this by feeling alone.