In normal mode, the player has to beat Future zombies at level 1.
Difficulty
Day 20 is a somewhat-easy level. The plants that upgrade from Cabbage-pult are pretty powerful, so defeating the zombies should be a cakewalk. The only thing the player has to worry are the missiles.
The small number next to the zombie icon means that zombie will always appear in that lane, with 1 being the topmost lane and 5 being the bottommost lane.
In hard mode, the player has to beat Future zombies at level 1.
Difficulty
Hard mode is drastically different than normal mode, the main difference being the player starting off with Kernel-pult and the presence of Gargantuar Prime. Gargantuar Primes can be devastating as you have to predict on where it will fire its lasers.
Waves
The small number next to the zombie icon means that zombie will always appear in that lane, with 1 being the topmost lane and 5 being the bottommost lane.
Lightning Reed has decent damage, but it won't always do arc damage on Shield Zombies protected by its shield.
Starfruit, being limited to targeting only a few spots, should be used with caution. Also try to predict where Gargantuar Primes would fire their lasers at.
Snapdragon's Plant Food effect should be used to finish the last few waves.
Gallery
Walkthroughs
Plants vs. Zombies 2 (China) - All by Oneself 2 One plant VS Zombies - Far Future Day 20 (Ep.135)