how to kill an alien

I am on level 3-3 and can not figure out how to kill the octopus alien. Please help.

When the octopus turns blue stop shooting it or you will increase his life. Shoot him only when he is red.

sAME A S above really make sure u wep is powerfull

I beat the game and to me the hardest alien to deafete is type P. Also the final boss is tough.
+ = super !

Also if you have a laptop you can have the plug in mouse and just click both mouses and you can kill aliens twice as fast!