This can only be completed on New Game Plus. Load up the Seattle Defense level and prepare to take on waves of enemies.
For the first wave, use your snow to create a line and destroy the enemies. Don't forget to refill your water in the ocean and while the first wave is ending you can build hedges in front of the buildings.
For the second wave, use your snow to take out most of the enemies and use your thunder on the enemies that don't die to the snow.
For the third wave, you'll be given a turret. Place it central and continue your strategy from the second wave.
For the fifth wave, you'll get a tesla coil. You can thunder strike it to cause an area of effect that damages all enemies. But, the tesla coil has to recharge. Place it center and continue your current strategy using the coil when needed.
For the sixth wave, you'll get another turret. Position it more toward the top of the screen and continue your strategy.
For the seventh wave, you'll get another tesla coil. Position it top-center and continue your strategy.
For the eighth wave, you'll get another turret. Move everything to the center. Wave eight is a quick wave before the final boss.
The final wave is the ninth. The boss is slow moving and with everything center you should destroy it in no time. Make sure to use your thunder and your tesla coils when charged.