TERMINOLOGY

Spawn - Show up on screen

Despawn - Leave the screen

Formation - Aircraft squad that spawns periodically

Boss - Final enemy that spawns after the aircraft requirement is fulfilled. Must be destroyed to advance to the next stage

Aircraft requirement - Minimum number of enemies that need to be shot down for the boss to spawn. Displayed in the bottom left corner of the screen

THE BASICS

All enemy spawns are based on the player's position. If you maintain the same direction, enemies will spawn in that general direction. For regular enemies there are some possible spawn points based on any given direction to make them harder to hit. For special enemies (stage 2's big plane and the bosses) you'll always get the same spawn point for a given direction and they only move horizontally.

Is best to move in a way to maintain the same direction while covering possible spawn points with bullets. I will refer to this movement as "sweeping". For incoming regular enemies use a very narrow sweep to stay in a general direction and, at the same time, increase the chances of hitting an enemy as soon as it spawns. Formations come more spread out, so go for a wider sweep at the time of spawn. Pick a direction where you can make the motion more consistently and go to town.

Another movement technique is to loop repeatedly. Can be faster but is riskier, not as reliable and more effective the closer enemies are to your aircraft.

"How do I know a formation is coming?"

Formations come (followed by a sound effect) after a specific amount of time, whether from the start of the stage or the last formation spawn. You'll be able to notice that enemies on screen start to flee and stop being aggressive to make room for the formation (more on that later). If the screen is too crowded you'll get a partial formation with no sound effect that won't give the 2.000 bonus points upon destruction. Shoot down the entire formation and all formations when viable. I find them easier to hit when spawning from the diagonals.

In this short video you'll find examples of the information mentioned above:

GENERAL TIPS

Taking down a boss with bullets (and not dying in the process) will trigger a cutscene that takes about 5 seconds plus the 2 second screen transition to the next stage. By taking down the boss with your own aircraft you'll skip the cutscene at the cost of a life. An ordinary death costs around 6 seconds.

A bonus life is awarded at 10.000 and then after every 50.000 points. So 10K, 60K, 110K, 160K... Paratroopers are worth points and increase by 1.000 for each one you get on the same stage, stopping at 5.000. So 1K, 2K, 3K, 4K, 5K, 5K... Also, stage 2's big plane does not count towards the aircraft requirement. Shooting it down is only good for points (1.500). If you're not counting on an extra life don't go out of your way for these targets.

Enemies will start shooting at you once they're on your back and after being on-screen for too long. To prevent that, avoid doing too many loops or turning to the opposite direction (especially when enemies are chasing you). Stages 2 and 4 are much safer this way.

There is a limit to how many enemies can be on-screen at once. If there is an enemy on-screen that is not shooting at you and is out of your firing range, try to despawn it. If there are too many enemies on-screen the formation may show up incomplete and the boss may not show up at all until much later. Bombs and seeking missiles may also influence the time it takes for the boss to spawn (if you have too many on-screen).

I find that by flying straight left or right, the boss will spawn within a shorter distance from you. If at the time of spawn you're angled up or down too much the boss will spawn much further away. If you manage to fulfill the aircraft requirement before the third formation spawns, the boss will show up quicker. Also works if you get a partial third formation.

STAGE SPECIFIC TIPS

STAGE 1 - Regular enemies will launch bombs that can be destroyed. The longer they stay on-screen, the greater the number of bombs they will launch. It seems that they do it more often when you fly straight left or right.

STAGE 2 - The presence of the big plane may influence the time it takes for the boss to spawn if the screen is already too crowded.

STAGE 3 - Regular enemies will shoot seeking missiles constantly. You can avoid, destroy or despawn them. I recommend avoiding them for as long as you can before destroying or despawning them.

STAGE 4 - Its safer to maintain the same direction (towards the top or bottom) for the entire stage when viable. The speed of your aircraft will increase considerably, forcing a adjustment in movement. Seeking missiles are shot just like in stage 3.

STAGE 5 - The enemy spawn positions seem to break the pattern of previous stages. Apparently, enemies will spawn wherever they feel like, even behind you. On top of that, they shoot a funky seeking missile and their bullets are bigger (both can be destroyed). Since this is the final stage you may shoot the boss down instead of crashing into it to save some time.

NOTE: All information above is based on game time and observation, not technical data or game code.