When you start the game, your fighter will be in the middle at the bottom of the screen. The alien spaceships have two ways they shoot at your fighter. They either shoot straight down or they shoot diagonally. You must take out all the alien spaceships on a screen. If you fail to do so, you will have to deal with their full complement again and your fighter will be set at the next higher level. If you happen to get killed during a wave, you only have to deal with the remaining alien spaceships.
Fuel is a precious commodity in this game. You may be able to let one or two waves of aliens escape but you will really be pressed to finish the entire round with enough fuel. If your fuel runs out, your game is over regardless of how many fighters you have left.
You can only have one shot on the screen at a time. This means you will need to plan your shots and don't fire unless you need to.
The first four waves of alien spaceships, come down in distinct formations. Understanding how these formations are set up is the key to taking them out quickly.
1) The Blue Aliens come down in a 'pyramid-type' formation. Starting from the bottom, the numbers are five at the bottom, three at the next level, two at the next level, and one at the top.
2) The Purple Aliens come down in two straight lines. With six in the bottom line and six in the top line.
3) The Green Aliens come down in a haphazard-type formation. They move back and forth across the screen and they are the hardest spaceships to deal with.
4) The Yellow Aliens come down in three rows with five spaceships per row.
As you shoot the alien formations, you will notice that the remaining alien spaceships will speed up. The key is to shoot them high up on the screen so that you are not desperately trying to take out the last ones at the bottom. This also means taking out the bottom attackers first to give yourself some 'breathing' room.
In addition to the alien spaceships, you must be wary of the bombs that drop down. When you are lining up your shots at the alien spaceships, always be aware of where the bombs may be heading and act accordingly.
On the fifth wave, you will have to deal with the 'GS' mother ship. Its shots are pretty easy to avoid and it must be hit dead center to stop it, collect the points, and refuel your fighter for the next round.
There is a 10k bonus trick. Normally, if you hit the GS mother ship with your first shot you get 950 points, otherwise you get 300 points. However, if you miss with your first shot and then destroy it with your 55th shot (counting from the beginning of the level, or when you start with a new ship) you get 10,000 points! There are 4 waves per level - 11 enemies on the 1st, 12 on the 2nd and 3rd, and 15 on the 4th (50 total), so you can miss 3 times (or hit bombs) before you reach the mother ship.