Gunstar Heroes

This boss can be slightly tricky with some weapons, for example the double homing weapon will split and half the shots will hit the legs causing no damage. The best way is to just up on top of the leg beside the main body and perform the slide attack which dishes out a lot of hurt. The guy shooting the lasers will hit you for a little damage but the boss will be dead long before he finishes you off (on normal anyway).

Alternatively avoid the lasers and shoot at the boss, eventually it will change what it is attacking with. When trys to jump on top of you, you can run/slide to avoid it, however should you find yourself caught underneath you can just jump out afterward. The next attack will be with bombs (which you can shoot to destroy) or a long spiked arm, which you can also shoot to knock back from you. Sometimes Pink will also dash cross the screen, you can jump to climb over it to avoid damage and here it's also a good idea to use your double jump attack to hit it for a lot of damage as you do so.

This achievement is easy enough, you'll unlock it by defeating the final boss of the first level, you'll come to find that in Gunstar Heroes you're able to almost mindlessly run through the levels holding down the fire button.

The only thing that really matters is which weapon you choose to use, I'd recommend either the homing bullets (the small green arrows) or the flamethrower weapons, the homing bullets will take out enemies without you having to aim in their direction constantly while the flamethrower inflicts the most damage, you also have the ability so save whenever you want, make good use of that!

* ^{Credit to SerenityWind the video}