Well, as I've said earlier in the CS, illusions that span the entire guild are relatively easy to break out of and/or short lived. In this case, the effects would last for about a minute, if that. Astaroth didn't invest much magic into the illusion because he wasn't affecting much, relative to what he's capable of. It was mostly just to create a pause in the fighting for him to leave, or to even break it up. Also, in the end, Perception Magic is completely harmless; any sort of meddling Astaroth did would be completely undone by the time the spell ended, in this particular case.