On this page of our guide to Batman: Arkham Knight we have provided a detailed description which explains how to defeat enemy tanks. From our description you will learn how to effectively get rid of enemy machines.
After leaving the building and eliminating some more enemies, open the gadget menu and choose Batmobile Remote Control. When you take control over the Batmobile, start from holding the key/button responsible for Battle Mode. Wait until enemy tanks appear in the area and attack them using 60mm cannon. Try to avoid the red beams by sliding and don't stay in one place for too long. You have to destroy all the tanks and park the Batmobile in the place marked by the game.