For all that, the process is quite simple. Find the steepest gravitic gradient around other than your own. This will be the field inside the enemy annie plant which is currently keeping their neutronium supply compressed. Now counter that field with a gravitic gradient of your own. The enemy's neutronium loses compression, and evaporates.
If you do it correctly the evaporating neutronium remains completely contained within the enemy annie-plant. If you do it incorrectly you might breach the enemy plant and be rewarded with an explosion, along with quickly-decaying radioactive gases. This is not a good way to shoot down enemy armor over your city, but it can be argued that there is no good way to shoot down enemy armor over your city.