Just assign a squad of battlearmor with TAG.
Deploy the Annihilator on an important crossroads, near a small inner city park, or close to the city border to fire outside.
Annihilators are, just like Gun Turrets, supposed to defend something important, preferably by sitting on it, figuratively or literally.
If an attacker can just ignore the annihilator for most of the battle, it's been miss deployed.
It isn't, in and of itself, a good city combatant. It's a great roadblock, however.