In addition to the above suggestions, I'd place the Tiger to the background or at the side of the dio, but not in the center. This way the Tiger wouldn't receive too much attention as a 'hero'. Furthermore, point the gun of the Tiger away from the knocked out tank. Make it look as if the Tiger just happened to drive past by coincidence, motto: "Hey, what's going on here?"
Position the KO tank pointing away from the Tiger to further separate the two subjects.
In the western world where people read from left to right, we are used to look at pictures and dios from left to right, too. If the onlooker sees the Tiger first (on the left-hand side of the dio), he would immediately associate the Tiger with what comes next. So put the the Tiger to the right to make it a minor subject and not to associate it with the destroyed target. Also, you can put the KO tank and the heroes on a slight elevation to emphasize the heroes.
I don't know if this makes any sense, but you can experiment with various positions. Make quick mock-ups of cardboard, for example, to study the effect or various tank/building/tree positions.