Project Starship

This unlocks after you kill the 900th enemy. It is cumulative across all your games regardless of character or difficulty. Thus, the game keeps track of your total kill count no matter how long or short the game, whether on hard or normal difficulty, and whether you select Garret or Gwen as your playable character. Accordingly, the achievement presents little challenge.

This is an achievement in the Exterminator series that one earns for killing 700 or more enemies. The series, as of this writing consists of five entries included in the third title update to the game. The Exterminator achievements I thru V in the 3rd title update pop at 700, 750, 800, 850, & 900 enemies killed respectively.

Accordingly, to get to the point to unlock this achievement, players will have become quite familiar with the game. To reduce its challenge:
—Select Gwen for auto fire
—Choose easy difficulty
—Press continuously to slow down the action
—Don't pick up H power-ups—there's one right at the beginning of the game. As @Moe GZ pointed out in the comments; they, temporarily, change the action to Hard Mode.

If you have just started the game, and have played continuously, the Exterminator achievements will unlock in order I through V.

However, if you killed more than 900 enemies before the 3rd title update came out, then this and the other achievements in the series for which you satisfy the unlock conditions will unlock as soon as you kill your first enemy. There may be a little lag, however, between the achievements unlocking and don't be surprised if they don't unlock in the correct time stamp sequence.