I think it's along the lines of what he said in a previous chapter about him having 0 attack power. Basically there's no way to overcome a wall you can never break.
Does anyone else feel like there's a page missing between 9 and 10? In the last chapter he had to chant the spell and then felt exhausted (might be other reasons for that part), but in this chapter there's almost no sign he did anything.
I'm guessing Eileen believed her sword would have been damaged or destroyed. Considering the damage done to the cliffside, if she tried to block Satsuki's hit it wouldn't have ended well, so she jumped to mitigate the damage to her sword. That's my two cents.
Oh, he only dispelled 10th tier magic as a commoner. You know magic so strong that the headmaster forbid them from using it? That's the person you should obviously make your enemy.