Be careful about staging a battle. Unless it's hand-to-hand combat, the scale will betray you. From the looks of your base, the belligerents are 70 feet away from each other. I wouldn't get my tank anywhere within that kind of range of that thing unless it was a complete surprise. It could capably destroy the gun from cover several thousand yards away.
I would also suggest that more than one vehicle (or subject) in a diorama gets dificult because the focus gets blurred. What am I supposed to be looking at?
I might suggest you build TWO dioramas- one of each. That way the dioramas each have a centerpiece, each tells a straightforward story, and scale isn't an issue.