Oh man, no one's helped the new guy?
I'll chime in. First stage and the first boss are straight forward, just dodge the bullets. Only things of concern on the boss are the green laser and forward shot.
Second stage, pick up the expanding force field midway through the stage and use it almost until the end with auto-fire or rapid pressing. This will get you through the entire enemy swarm. Make sure to pick up the spread shot right before the boss. At this point you should have three powerups that speed up your power shot recharge (IIRC). Move in to the turtle head so your pod touches the boss and the minute you see it flash, and cause damage, release the shot. Continue to move and fire while at the same time having your pod on the boss's head, this is the fastest way to clear it out. Once it's destroyed and goes into its shell, you can just stay all the way at the bottom left corner. If you had the right timing he will not be able to touch you and you'll get out of the corner right before the scenery rock crushes you. At this point you just need to dodge one onslaught of the sparkle rocks/scythes/enemies, and be able to destroy the boss before he spreads his "wings" and hurls lasers.
Third stage is all about pattern recognition (the entire game is) and seeing which scenery is safe to fly through and which parts will destroy you. Once you pick up the backwards tracking laser in the middle of the stage, you just stay ahead of the boss and let the laser do its work. Once the boss comes back around to the front, concentrate on destroying its satellite head quickly, making sure to stay above it so he doesn't whiplash you, and then look for the sparkle in the eyes to avoid the mouth fireball.
I'm obviously not using any of the proper terminology for all of these weapons. I know BBH or Gemant will destroy me
Best thing to do is grab a 1CC replay off YouTube and study the patterns. I did that a while back and was able to get to Stage 5 on one life, stage 7 boss on the second life and after a few tries make it all the way to the last boss (where I gave up after about 10 continues). With this particular game design and checkpoints, it's vital to not die as you lose all of your "weapon recharge speed" powerups, which make all the difference in the game since the regular pea shooter forces you to rely on an incredible amount of bullet dodging, all while having an absurdly large hitbox.