I don't see any limitation on the number of 'Waiting' spells she can cast. Granted if its an illusion, then its not mobile. If she's casting a Phantasm of the Waiting variety she could only give it a simple command - such as attack or fire at anyone who enters. I'd think a few phantasms could attack a person, but they would probably all pile on the first person entering an area rather than spread out. And I think the GM could limit the number that can effectively attack one person at a time. Now if she created a horde of archers then they could all unload on the first person. Or form a shield wall and poke anyone that attempts to pass would be pretty effective.
I don't think this is exploitation, given that without the Illusionists control via concentration its just limited to a simple command.
Mal