Grass Effect is an achievement in Plants vs. Zombies 2. It requires the player to spend 500 sun on attacks, which is equivalent to ten taps through the use of Magnifying Grass. It is worth 10 points on Game Center for iOS devices and 500 XP on Google Play Games for Android devices.
This achievement can be done easily if you generate enough sun quickly enough and use Magnifying Grasses as your only offensive plant. A good level for this is Far Future - Day 21, as there are four red square Power Tiles in the three backmost rows where you can plant your Sunflowers or Twin Sunflowers for maximum effect. It also works perfectly with the fact that you have to make at least 5000 sun and can only have a maximum of 16 plants at a time.
Another good way to achieve this is to play higher levels of Endless Zones where Gargantuars are usually present. It requires at least seven hits for them to die. Make sure there is enough sun in the sun bank so that the Gargantuar does not have time to throw its Imp in case no more shots can be fired.
Last Stand levels may be used for this purpose during the setup phase, but not for actual gameplay. Nonetheless, the achievement requires 550 sun minimum, and every Last Stand level will give you more than that.