There's supposed to be 2 aliens but there's 3 and ther'es no way to fight them

The aliens make a shield on their side that lasts about 6 of your shields. As soon as you land make 3 shields in a row, then move a centimeter forward, only once, and make another 2 to 3 shields. keep firing at this point. Their shield should vanish and you will hit them. The reason you need to move just slightly forward once is, if you don't they will move towards you and get you. So move only once, and they will stay put a little longer for you to get them the moment their shield fails. Hope that helps I've done it a few times in a row now. Good luck, it just take s a few tries, you'll get it.