Space Invaders Infinity Gene

You only need to beat the last level to "beat normal mode" so once you have finished the game on easy go to settings, change the difficulty, & go & replay level 5-6

To make this quick, play through all the levels on easy difficulty first using your weapon of choice. I found field to be most effective, especially when powered up with some tricolor orbs but i found some of the bosses, like the crab for example, were easier taken out using the wave or lock on weapons. Once you have unlocked all the levels you can start up the other difficulties (normal, hard, insane) and play the final level, 5-6, to unlock those achievements too. On the final level, the field weapon makes mincemeat of the boss there no matter the diffculty. Setting your stock to 9 is also a help. Skipping the credits does not affect achievements.

Change the difficulty and stock in the options menu from the main menu and then go into play modes to play with your selected options.

Eventually you'll unlock the ability to have 5-7-9 stock ships to utilise which makes dying a bit less painless, but on easy mode this really shouldnt be an issue for you. This is also helpful for challenge and bonus levels.

To unlock Hard you need to beat the game on Easy or Normal. To unlock Insane, you need to beat all of the bonus levels up to 2-6, which arent too painful. To unlock Nightmare, you first need to complete all of the challenge levels.

These are not stackable.