Offense: These should be used like any artillery unit. They have good range and decent damage and pretty good health unless they are dealing with heavier tanks or helicopters. Keep towards the rear of your attacking units for the best effectiveness but at least they are cheaper than most dedicated artillery units and have a pretty quick recharge so losing one isn't the end of the world.
Defense: These can be deadly if they get behind a wall of units and fully promoted are a threat to Hastatus Snipers. It is best to send a fast tank with high health like the Reaping Truth to kill one of these or use helicopters or even longer ranged artillery with support. Rushing it with two faster light tanks would work as well since it has only an average rate of fire.
Variant Specialty Edit
This is the long range variant