Flower Power

Difficulty: Hard-Very Hard (Easy-Medium if Survival levels)

Note: If one Twin Sunflower out of the ten gets eaten, crushed or stolen by a Bungee Zombie, blown up by a Jack-in-the-Box Zombie or killed by basketballs or peas fired by Peashooter hybrids of zombies before the end of the level, this achievement would not be counted. You will need to have ten Twin Sunflowers alive by the end of the level. Playing Survival: Endless also won't count because this level doesn't end.

Strategy
Play a Survival Level (not Endless), preferably Survival: Pool, (due to it being a level in the daytime that has six rows) 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 (a Tall-nut planted in a Pumpkin gives enough time for the Twin Sunflower to recharge), while you gather the Sun required to upgrade all of your Sunflowers into Twin Sunflowers. Using this technique may also earn you the Sunny Days Achievement.

Another possible strategy is that you can still plant the Sunflowers and upgrade them into Twin Sunflowers after you defeated all of the zombies in a level. Once you beat the level, do not pick up the item dropped by the zombies at the end of the level. Make sure you have enough Sun at the end of the level, because the Sunflowers/Twin Sunflowers will stop producing Sun once the zombies are defeated (sun will stop falling from the sky too in Day levels). This will take a while as to the Twin Sunflower's slow recharge time. The best thing to do is to upgrade the Sunflowers into Twin Sunflowers after you defeated all of the zombies, as the Twin Sunflowers will be alive at the end of the level, without any zombies attacking it is an easier strategy.

Trivia

 * The Trophy is the Gold Sunflower Trophy with two heads.
 * This is because Twin Sunflower has 2 heads, and it plays a major role inside the Achievement.