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.
o Don't miss the alien smart bombs as these clear a good few buildings in advance as well as awarding points.