So, I'm used to exploit it to farm money (as there are two endless lines of crystal-carrying ships, after the boss). But sometimes, the crystal-carrying ships didn't show up, instead there's only endless amount of formations of three annoying, spade-shaped fighters, or after the boss' dead, the stage ends.
Anybody knows what actually can trigger this stage endless?
not sure, but the asteroid? level where the level stops and the gunner starts popping those little gray orbs, if you sit in bottom left with sonic lvl 4 or something and weight the mouse, you can break the game with the amount of cash you get. You'd get set for the entire game from 10 mins of it.
Don't forget the Stargate Boss in episode 3: Just stick to the bottom left (or bottom right) and just keep destroying the small brown bullets which home in on you. Any weapon which goes straight up is recommended (those at a forward angle, like the Multi-Cannon Front, Hot Dog Rear, Rear Heavy Missle Launcher etc.) may destroy the boss, unless you find a way to prevent this...)