If you spend too long on a level without completing it, you will hear a sound effect, and the music will speed up. The enemies remaining on the screen will then go towards the surface (the top) of the level. Get up there before them, wait for the enemy to surface, and then use your inflator to catch the enemy, and inflate it until it pops. The achievement will then unlock.
Alternately, just kill all but one enemy on level 1, and then wait at the top of the screen. Sometimes the enemy will actually follow your dig paths to the surface if you're up there, if the enemy is free to move (i.e, not in its own 'segment')