Nom Nom Nom

Difficulty: Extremely Hard

Note: If you grow any other Plants, even if it doesn't kill a zombie, you do not achieve this (e.g: if you plant a Tall-nut or Pumpkin).

Strategy
The best way to get this achievement is on any of the Daytime levels (not Pool or Roof) where the number of zombies is low, the fact that you won't need extra plants (Lily Pads, Flower Pot), and also because most of the zombies are effectively the same, due to the fact that the Chomper is the only offensive plant you can use.

Pick the Sunflower, Wall-nut and Chomper (pick any other plant, but do not plant them). As the achievement states, you can only use 3 types of plants to win. If you plant another plant, you can't achieve this.

Trivia

 * The Chomper's head on the icon seems to be mishapened.
 * However, that's the shape it forms when its swallowing a zombie.
 * In the description, it says Wallnutinstead of Wall-nut.
 * The name Nom Nom Nom is probably based on the one of many ways of expressing eating with words.
 * This may be a also reference where Wall-nut and Chomper were roommates in college, according to the Tree of Wisdom.
 * The lawn ornament in the Xbox live version is a Wall-nut, Chomper and Sunflower totem, carved out of wood.
 * You cannot get this on 5-5, even if you do not plant any Pumpkins or Cherry Bombs. This is because there are Flower Pots.
 * Technically, this may be uncertain, because you are not required to plant any, as some start out on the roof.
 * If Flower Pots are not allowed to be planted, but you can use those already on the roof, it is possible to do it in 5-1 the first time instead of 5-5.