Here are some special tips for beating the big bosses. The first few levels don't really have any strategies--just throw tons of firepower at them. But starting with level six, things start to become more difficult.

LEVEL 6
	Since the water-gun can track you, take it out first. There is a single spot you can stay, perfectly in line with the water-gun, but behind the firing path of the missile guns (see picture), where you cannot be hit. Kill the water-gun. After that, back way off and duck low, and take out the missile guns from below their firing range. After a while, planes with double-bullets (green weapons crate) will come out, and you can use the angled bullets to take out the last little gun (the one that only fires straight ahead), while staying out of it's path.
LEVEL 7
The Red Baron Wannabe flies in loops and figure 8's, so that leaves you only a little circular area near the left side of the screen to safely maneuver in. The key here is that the more contact your superweapons have with the plane, the more damage they inflict. You want to use things like FireWalls and ThunderStorms when the Folker is moving away from you (towards the right), so the firepower stays on the enemy longer.
LEVEL 8
	The Spitfire is your weapon of choice here. As the Laser Carrier moves upwards, it pauses once in the middle of the screen. Halfway during this pause, execute your Spitfire. By the time your bullets come flying out, the Laser Carrier will be moving downward again, and every bullet you send will make contact with it.
LEVEL 9
Forget the Passenger Liner, the Jumpers, and the Fuel Plane. The only thing that matters is the little hose the Fuel Plane extends when refuelling. Destroying it will destroy the Fuel Plane and weaken the Passenger Liner to the point where you will be able to destroy it. Your best bet is to freeze the planes when the Fuel Line is exposed.
LEVEL 10
	This plane jumps forward at you and recoils back. Just like Level 7, shoot a FireWall when the Folker is moving backwards, as that will maximize the damage inflicted. Don't expect it to die the first time you kill it, either.