The "Rolling Thunder" level is perfect for this. You'll start off with two tanks to assault the hanger with. Once you've taken the hanger, and defended it from counter attack, any armor you've lost will be replaced and you'll be given an order to take out the enemy armor in the ruins. Once this part of the mission begins you'll have an unlimited supply of tanks, if one is destroyed, another will spawn back at the hanger. Either let the AI drive forward, or if you find the AI is dying too quickly, take a tank to the front yourself, let it get damaged, heal it, then either let the enemy take it out or destroy it yourself, let a new tank spawn, rinse and repeat, and you should get this in no time at all.