To play as 8 androids, you will have to progress far enough into the game to have beaten Venom boss. This will unlock the final Android you need to unlock this achievement.
In order after each boss, you will unlock the following.
These androids all have achievements tied to them regarding their weapons. The boss fights are very hard at first, as your battery constantly drains unless you get a green battery pick up. My best advice is to use Starch. Her laser attack is even able to land hits from the other side of the levels. Her secondary attack is also vital for staying alive in crowded rooms. Very useful on boss fights too. You may find others are better, but that's my take on making it easier.