You dont have to have all of your squad slots filled, you just have to take all females (You could take just one soldier).
This achievement is easier the sooner you attempt it. Even as early as your second mission, where there are only 4 Sectoids (easy/ normal). If you kill a sectoid that is Mind Merging it automatically kills the other.
So i would suggest 2 females + 2 grenades = 4 Death