Shooting Star

Complete Seeing Stars before the first Flag Zombie appears.

Strategy
It's very hard to beat Seeing Stars without letting the first Flag Zombie appear. You should pick the following (up to 8 slots):

Sunflower

Starfruit and Imitater Starfruit

Twin Sunflower

Kernel-pult

Squash

Puff-shroom (do not wake them up)

Sun-shroom (do not wake them up)

As the achievement states, you must complete Seeing Stars without letting the first Flag Zombie appear. If you're not good at collecting enough sun, bring in your Twinflowers and then use the Starfruits and Imitater versions before the flagger appears and you're done!