I recommend getting this on level 2.
There are 5 enemies on round 2, but the reason why I recommend round 2 instead of round 1 is that the placement of the rocks is much better than in round 1.
The first thing you want to do is 'release' the enemies, so that they're all in one huge, open pathway. Eventually they'll dig through the ground and come after you, but it's best to try and release them early for positioning purposes.
I recommend trying to use the top left rock to do this achievement.
When you have enough enemies following, try and do a long vertical column up below the top left rock, but don't go right up to the rock yet. Make sure the necessary amount of enemies are following you and are pretty close, so there's no side passages for them to escape to. When you're sure there's no escape for them, go right up against the underneath of the rock, then move to the side. The rock will fall, and hopefully kill four of the enemies.
You can see this method in my video below:
Also, don't forget that you can choose level select in the settings before the game starts. As long as you've reached level 2 in a game, you can level select it.