Achievement is easy to get just go forward and take out the enemies that deliver a large payload of bubbles. If you get surrounded simply switch to a different bubble. Keep going forward.
For all you whiners re: "health bars & random upgrades" check out the name of the game. It is called RETROSHOOT not KINDALIKEARETROSHOOTERWITHSOMEMODERNGAMEPLAYELEMENTSADDED.
 "Bubble Tanks"){kind=icon}
 "RetroShoot"){kind=icon}
 "Red Moon"){kind=icon}