How do you get the monster masher achievement in the ipad version?

Once you completed the game you can go another run, but then with all plants unlocked from the start.

At an early level take pumpkin, Tall nut and the regular wall nut. Fill the top lane with those and play the other 4 lanes regularly.

This will pile up the zombies behind your top lane wall and then you can take them out with a single squash. You can do something similar with a cherry bomb for an other achievement.