Gunstar Heroes

This achievement is easy enough, you'll unlock it by defeating the final boss of the second 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 catch is that the final boss of this level is can transform into 7 different bosses, which means unless you're playing on the Hard mode you're gonna have to hope he transforms into "Crab Force", I actually managed to get him to appear on Easy mode using a very easy method. Once the level starts proceed through it fighting the enemies on the train until you reach what looks to be the front of it, you'll know it's the right section because there's an enemy in there wearing red. Start making saves every couple of seconds here while you tackle this part of the level because the boss is next up, doing this will mean that if you don't get "Crab Force" to appear you can quit out and then load up your game right where you were, just keep doing this until you get him to appear, just make sure you make the save before the game warns you that the boss is incoming.

Once you get him to appear make a new save, that way if you die you can just reload and keep trying until you beat him, he's easy enough, I just got semi-close and took him down in no time at all using the flamethrower.

Lastly, 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!

This boss has seven possible forms, however on easy only three of them appear, yet 5 appear on normal giving you a better chance of facing the crab form. (All 7 appear on Hard however the boss becomes tricky on the hardest two settings) It's one of the harder forms as timing your jumps and where to stand can be awkward.

The first type of attack will send blocks along the upper or lower portion of the screen that rise to walls which can only be avoided if you are on the other side. Quickly double jump to move to the opposite side that they appear on.

The next attack sens balls of light diagonally across the screen. Either stay on the side that they appear (ie if the upper portion of the crab generates a shot it will hit the lower portion) or move so that the ball doesn't hit you (they strike the same position each time)

The final attack sees mines laid down on both lower and upper portions of the map. These you can simply jump over and are the easiest to avoid.