Generally, whack zombies with the mallet by clicking on them or by tapping on them if you are using a touch-screen device. Some zombies are harder to kill than others.
Use sun to plant Grave Busters to destroy graves. Graves spawn frequently closer to the house after halfway through the level. Each grave carries a little treasure or two, so try to destroy as many as you can.
Use a Cherry Bomb if too many zombies come on the screen at the same time (maybe in the final wave). Potato Mines are not useful in this level, although they only cost 25 sun.
Get rid of all graves except the ones that are clustered together. Tapping around the cluster will get rid of most of the zombies.
Get rid of the graves closest to your house, giving you more time to destroy zombies before they eat the player's brain.
Spam clicking on a grave before one that is being eaten by a Grave Buster is not a bad idea, as zombies may come out of it, eating it.
Zombies will never spawn on the grave that is being consumed, though.