This is a very simple to get achievement but just requires 2 controllers (A Gamertag for each is required) and a bit of setting up if playing on your own...
First start up Multiplayer with 2 controllers and select the game mode to play co-op, and then the first level on easy.
Regardless of who selects Water the other will also get the achievement but in this case we are going to give Water and any other element to your main control and then give the remaining 2 I recommend Wind and Fire to player 2.
Now if you are on your own set the player 2 controller at your feet and have your toes hover over A and B so you can use the Fireball and Lightning.
Let the guys build up a tiny ramp so you have room to let them gather onto and line up your player 2 into a set position where the crosshairs are set onto the path (You are going to be unable to move this control around to activate the other powers due to time constraints) and then have the player 1 crosshairs aim over the same spot (Should have a glowing light on your screen)
Now with player 1 use rain to slow down the guys and create a batch in a small area the second you have gathered some use your Freeze attack and then roughly 1-2 seconds after press A and B on Player 2 to smash down onto the frozen little guys.
This should take roughly 3-4 attempts to collect 30 but is very simple and will pop the second 30 have been killed in this manor.