You probably already know that this achievement is bugged. What you should know, you can still get it with about 1 / 1,5 hours of grinding.
There are only 4 things you need to know:

1. The Killed enemy counter always resets when you start up the game, so don't quit until you are done with the achievement.

2. It doesn't count your total kills only the kills on a specific stage, that means you have to play the same stage over and over again.

3. Splash Hill Zone 1 is probably the best choice for this, If you go on the optimal route you can kill 25 enemies in one run that takes around 1 minute.

4. There isn't an in-game counter for the kills but you can see the numbers in your data.txt which is in the main folder of the game. (By the way you can't change the values so cheating is not possible.)

The optimal route for Splash Hill Zone 1 is as follows: