I was in the second row and seat 3. The moment we left the training room I hit the aliens coming off the ship until my vehicle turns and then I hit the high window shots the rest of the way. Once I zone in on one I stay there until I can no longer reach it. I hit windows the entire way until you reach the photo op. Once there I just pick an Alien and stay on him until were done spinning. I then pick a set of eyes and repeat the process. Once I hit the fusion exhaust port I don't go straight for it but rather I hit the targets on the wall until after we clear the support beams. Once we get past the support beams I stay on the exhaust port.
If you have a good gun this is all you need to do to reach 999,999 before you even get the bonus. I hope that helps.
Btw I still got the bonus just so I could say I did
My score was max and the vehicle average (6 total people on board) was 274K and change. I destroyed!!