Basically a Scramble clone with a few variations, Cosmic Avenger has you in control of a space ship armed with missiles and bombs, of which you can fire one of each at a time. There is also a radar scanner which shows the position of alien UFO's and tanks. You must make your way across the alien landscape destroying all enemy targets.
On the first stage you encounter alien missile launchers, which fire missiles vertically at you when you get close to them, and also diagonal launchers which let fly from below you, and chase you from behind.
Once you clear the city landscape, you are faced with enemy tanks moving across the ground which fire missiles at you at regular intervals. These missiles must be shot or avoided. All the while on both the city and tank stages alien UFO's appear both from behind and in front of you. They hover in front of you firing bullets and must be either shot or avoided. When they appear behind you, they fly across the gameplay area to hover in front.
Once past these two stages, you enter the submarine stage. Your ship goes underwater and must face the enemy submarines, mines, depth charges and underwater missiles. If you survive the underwater stage, you enter a new city stage, armed with more missiles and populated with greater numbers of enemy UFO's.
Alien UFO's can be destroyed by missiles or by contact with another UFO.
Maximise your score on the tank stage by moving as slowly as possible. Tanks will appear as fast as you can fire. Remember to destroy any missiles fired by the tanks.
Bomb diagonal missile launchers when they are below you, before they have a chance to launch.
Vertical missiles are fired before you reach them. Slow down and allow them to fire, then destroy the launcher before it reloads.
Destroy launchers situated on towers by firing missiles at them. This will clear the way for you well in advance.
Watch the scanner for any UFO's coming from behind to see if they are coming in low or high. This will enable you to avoid them, but watch out for bullets.
On the underwater stage destroy the mines so that alien subs are blown up in the subsequent explosion.
You can move above enemy subs and bomb them safely as they only fire horizontally. Watch out for depth charges falling from above though.
Don't miss the alien smart bombs as these clear a good few buildings in advance as well as awarding points.