1) Start the game.
2) Select Single Player, then Survival mode.
3) Set cooldown time, wave time and difficulty to minimum.
4) Set number of waves to max. Should look like this:










5) Click play.
6) Just wait few secs, the wave will end before it started, it's that short. The result is kills + headshots = achievement spam.
7) Keep waiting till you see that 1 enemy (boss) spawned. Press ESC and exit the game.
8) Then click continue to start a new game.
9) After a while Steam overlay will go down and you will get the remaining achievements.
10) Uninstall forever (hopefully).