Flower Power

Note: If one Twin Sunflower gets eaten, crushed flat, stolen by a Bungee Zombie, blown up by a Jack-in-the-Box Zombie or hit by basketballs or peas fired by Peashooter hybrids of zombies, this achievement won't be counted. Playing Survival: Endless also won't count because this level doesn't end.

Strategy
Play a Survival Level and plant Sunflowers to be upgraded to Twin Sunflowers. It is recommended that you protect them with Pumpkins and Umbrella Leaves. Replace the Pumpkins when and if needed. Of course, use your usual Survival strategy for that level. All 10 Twin Sunflowers should make through to the end. Or you can play Zombotany 2, because there are three flags. It's better to use six Gloom-shrooms and two Garlics and then keep 10 Twin Sunflowers to alive and don't let them get shot down by peas.

Alternatively, it is possible to replay level 1-1 and hold off the last Zombie with Tall-nuts or other defensive plants, while you gather the Sun required to upgrade all of your Sunflowers into Twin Sunflowers.