His art won't be targeted, but it also won't be deliberately avoided - which is to say, they won't make actual attempts to destroy it, but if it's in their way or accidentally destroyed they won't care.

As for sneaking around, it will take the Null longer to catch on to stealthy or more subtle types of resistance, but they would eventually, and would then put those people on their hit list (if they caught them doing it) and/or begin to be more careful & guarded in whatever areas they were affected (if they didn't catch anyone, and only realized things were being targeted). However, you could certainly get a few things done before they caught on!