Immortal

This achievement is earned after surviving 20 flags (waves) in Survival: Endless mode. Once you have completed 20 waves and selected your plants for the next round, you'll then have the achievement completed, since it won't be awarded until you begin the next round.

Almost every properly-designed setup can make it through twenty flags of Survival: Endless, as the waves only start to become large when you reach 30 flags or more, especially after 32 flags, Gargantuars appear before flag 33. This is similar to the achievement "Alive and Planting" for the Xbox Live Arcade and PS3 versions, which requires surviving 40 flags.

Trivia

 * It is an easier version of Alive and Planting.
 * You'll get this achievement only if you resume at the start of 21st wave.
 * If you discontinue after finished 20th wave without resume to the start of 21st wave, you'll lose this achievement.
 * This is not true, you'll still get the achievement if you head to the main menu, (thus saving the progress) and go back and select your next plants.
 * The picture looks like a flag with a twenty on the left top corner as it's the twentieth wave.