The main story of Dave The Diver is quite long, and it starts with you trying to get different kinds of fish for your Sushi Restaurant. Eventually, you meet Dr. Bacon and start looking into the sudden appearance of Sea People and the reasons for constant earthquakes in the Blue Hole.
After helping a few Sea People, you'll go through the Glacier Passage to reach the Control Room where you'll have to battle the final boss to save Sea People and stop the earthquakes. While you're trying to beat the game, you can do some specific things to unlock the Dev Killer achievement.
How To Unlock Dev Killer Achievement
Once you beat the final boss and party at Bancho Sushi with everyone you met along the way, Dave will decide to have a good sleep for once. In sleep, he'll dream of getting on a Spacecraft and heading into Space. This will trigger a mini-game called Space Diver, and that's where you'll get this achievement.
Space Diver is the credit screen of Dave The Diver where you'll see all the developers on the left side, but there will also be a few developers on the right side along with Dave. You can control your character to move around here and also shoot with your Space gun the same way you shot guns underwater.
On top of your screen, you'll see the score and the number of developers remaining in Space. To get the achievement, you simply have to shoot down every single developer name before the credits on the left are done. When the game starts, you'll see the names scattered in an oval shape, but don't worry about remembering them.
You can simply move in any one direction and the developers will automatically keep appearing on your path, even if they weren't there at the start.
If you weren't able to get all the devs on the first try, you can play the game again through the Minigames app on your phone when you're on the surface. The achievement will unlock as soon as the game ends given that you got all the devs and the counter at the top reached zero.