SPOILER******
Here is my list of tips...
although a lot of the good tricks take 2 people or so to achieve.
the first trick in the barnyard, hit the fox atop of the small hen house of front, you'll see chickens running out work 1,000 points.
Another in the barnyard is hit the 1,000 point mouse scurrying up the barn. if you do that, the barn will flip with 2,000 point mice inside, if you get all of those, then you'll see 1,000 point mice pop up all over the screen.
2nd game is bo peep.
hit all the balloons in the clouds, it will rain 500 point balloons with each cloud popped, if you get both clouds at the same time, it will rain 2,000 point balloons.
I always keep going for the 500 point balloons up front and the 1,000 in the back.
3rd is the Plates.
just be quick and get as many as you can. you'll see the jumping 2,000 plates in the back, if you and your partner both get those, the hill will drop and a tank will start shooting 5,000 point plates at you.
4th aliens
right away as fast as you can, try to get a ring around all of the aliens before they pop back up. If you achieve this, the ship will take off, and a monster will appear with its mouth open, aim for his mouth to rake up tons of points worth 500, 1000,2000. between the monster opening its mouth, aim for the 5,000 alien in the corners.
last is the targets with the darts. hit all the targets once, to reveal a screen full of 2,000 point targets.
the last scene, get all the mine cars, aim for the bat atop, he will turn around and reveal a 5,000 point target. the very end shoot like you have never shot before.