Quick tips

Try to destroy the appearing enemy at once with a burst of shots. This way it can shoot only 1-2 laser beams at you, and you get additional money for the fast kill.
Try to keep the number of enemies on the stage low. It's much harder to deal with 5 enemies than with 1-2.
If you can't overwhelm the enemy, shoot at them anyway as much as you can, becuase every hit charges your specials, even if it only hits the shield.
Avoid your own bullets if you have upgraded the lasrer damage. A few beams shot at yourself can destroy you!
If you have activated the force field, it defends you against the shredding damage when your hull collides with the enemy.
Try to finish the first stages with high accuracy and shadow dancer bonuses. This gives you a huge upgrade advantage for the later stages.
Upgrading your engines can help you achieve large shadow dancer bonuses.
Try to kill your enemies one by one, depleting the selected saucer's shield and then taking in as much damage as you can.
When you have activated the EMP, you can use the frozen saucers as shelters from the newly appearing enemies.
Thunderbolt renders all enemy shields zero. Use this to attack the stongest enemy, hitting directly its hull, not letting it recharge its shield.
If you move around, the enemy tries to predict your motion and shoot in front of you to hit you, so it's better to move more unpredicably.

This is an old web browser game.