I found this easiest on the last stage of level 1 (1-10 I think), where the plants come up on the conveyor belt. At the point where "A large number of zombies approach" as indicated by the game, I had a few wallnuts pretty close to the far end of the lawn where the zombies start approaching from. There was a big bunch at the wallnut and some around it on the other parts of the lawn, I dropped the cherry bomb on them and killed plenty, unlocking the achievement. May involve a bit of luck but should be fairly easy this way, theres a heap of zombies around that wave. Hope this helps!