They had some freak day a while back where the ride go stuck on the slow setting and all the highs were over 2.2 million. But then you had like 9 mins to shoot at a lot of targets. And you can also get freak scores if you get ride stopped in front of one of the Zurgs and you know the secret.
So, to hit the target, look at the Z where his chest plates meet and go down about 6 - 8 inches. Once you see it, you will never miss seeing it again. And remember, you can get lucky and unlucky with the first Zurg as his is turning and will face some space cruisers and not really face other cruisers.
And now Im tempted to take a green laser and tape it to the sights along the top of the blaster so I can see where I aiming and get the points better than you... :P