Kingdom Rush

In the 9th Level of the Campaign, "Stormcloud Temple" there is a statue/obelisk in the bottom left corner, above the Fireball and Reinforcements powerup. You can navigate over to this and add a sorcerer for 100 gold. After adding a sorcerer, the statue will begin to glow and once it has been fully charged, you can click the target icon and target an enemy with it. Do this 20 times during the level to unlock this achievement.

Adding more sorcerer's (up to 4) will allow you to charge the sunray faster. Please also note that you will need to target an enemy directly with the targeting reticle, otherwise it won't fire.

If I can explain anything further, please don't hesitate to ask!