You only need 50. If you get every EVIL EYE in Chapter 1 through 4, you will have 51!
Most of them are pretty much out in the open, or in obvious "hidden" spots. You can replay levels, so if you're missing one and can't figure it out, check the VIDEO for each Chapter!

There's 75 total! So you can easily unlock this!
*Note are four separate achievements for destroying 5, 25, 50 and all 75*
Please mark +1 THUMBS UP! I don't do solutions very often!
CHAPTER 1 (13 Evil Eyes Total):
*** Spoiler - click to reveal ***Chapter 1-1 - Tutorial (NONE)
Chapter 1-2 - Anotherland (NONE)
Chapter 1-3 - Sea of Sand (5 Evil Eyes)
Chapter 1-4 - Black Rock Canyon (4 Evil Eyes)
Chapter 1-5 - The Crossing (4 Evil Eyes)
CHAPTER 2 (17 Evil Eyes Total):
*** Spoiler - click to reveal ***Chapter 2-1 - Roots (3 Evil Eyes)
Chapter 2-2 - Into the Wild (5 Evil Eyes)
Chapter 2-3 - Father Oak (4 Evil Eyes)
Chapter 2-4 - Tomb of the Giants (5 Evil Eyes)
CHAPTER 3 (13 Evil Eyes Total):
CHAPTER 4 (9 Evil Eyes Total):
CHAPTER 5 (9 Evil Eyes Total):
CHAPTER 6 (9 Evil Eyes Total):
CHAPTER 7 (5 Evil Eyes Total):