I got this by pure dumb luck in Acid Man's stage on Newcomer difficulty.
Just go into Acid Man's stage and you'll encounter at least 2 Shpiders at the very beginning. I easily killed the 1st one, but the 2nd released 2 Mini Shpiders when I destroyed it. I moved over one opening in the floor and one of the Minis jumped right into the acid pit and died. BOOP-BEEP! Easy achievement.
Good hunting!