Sorry for being too agressive in my first reply, I got mad because I couldn't beat the 7 hard level but after a few tries I could finally beat them!
So, about the Sky Turtle that I can't unlock, it's the one from the "K.O Base" from the gallery, it might be a problem with the browser version because I'm playing it there because I also can't see the rest of the aliens that are on the screenshots of the game page of itch, you know, the new aliens. So, to unlock the Sky Turtle I think I did everything that's supposed to be: I've lost in the assault mode where you unlock the other two human chicks, I've lost to bosses in the normal levels, lost to bosses in hard levels, lost to normal enemies in both hard and normal levels and I think that's it.
Also here's the screenshot and also sorry for my bad english, it's not my first language.