Resogun's 5th level is what separates the men from the boys. This is the first level where you absolutely have to use warp speed to survive. You really learn a lot having beat the 5th level compared to having beat any of the others.
I finally got to the end boss and i've found you actually want to use your boost very aggressively to clear as many enemies as you can then maneuver around fending off stuff for your life while your meter recharges.
A general tip for this game is that you want to rescue as many of the green dudes as possible because for every green dude you don't save, youll have an extra speedy enemy you'll have to dodge and you won't get a powerup like shield, weapon upgrade, points or extra life.
You mainly have to be ready to kill the enemies that release the green guys from the cages. If you let them escape after they spawn the green guys die so you have to hunt them down. I've found that my Astro A40s help me a ton hearing where they are. So i'll know whether I have to zoom halfway across the map to killsomething or kill something a little to my left or right allowing me to pay closer attention to my immediate surroundings. That said, I believe the enemies always spawn in the exact same locations so you could just memorize your attack plan.
The 4th boss is actually the easiest in the game since all you have to do is kite him although if you don't shoot backwards while constantly warp speeding away from him then he'll kill all 3 of your lives in seconds.
RIght now i feel like I could take on the whole empire myself. "HEY IMPERIAL FLEET! GET READY TO SUCK SOmE DAK!" This game is the best.