It's a great idea making the player choose an upgrade for the enemies between rounds, made for some interesting decision making and fits the theme really well. I liked the slowmo when you are aiming, I would consider adding a 2nd jump if the player selects the alien while it is in the air, so then the player can redirect the attack (probably using the slowmo again), I think it would lead to some more exciting gameplay. But it's still great as it is and it looks amazing