try running AIscripat and make the script u run one that is devoted to attacking towns such as [terran town A]
they should aim for the building because one thing the computer knows is if u loose all ur buildings "u die"

ADDITION:
that might not work but its worth a try. overthwise use a trigger ordering them to attack supply depohs at this location...but they wont attack anything other then the supply's even when they are gone,,,,or so i think