Types

There are different Pokemon and Attack Types in Pokemon Tower Defense. All have advantages and disadvantages over the others. It is similar to a giant game of rock paper scissors.

Water Gym Challenge
To win this the level Cerulean Gym, and Cerulean Gym 2, you should probably use Grass or Electric type attacks.

Rock Gym Challenge
The rock gym is Pewter Gym, and there is a rock gym challenge Pewter Gym (challenge).

To win the gym and save Joey, you would be best off to use the Grass, Water, Steel, or Fighting attacks.