I myself has found almost all of the skulls in the game. I know where they all are but some i just can't get. Remember, you have to be playing Normal, Heroic, or Ledgendary to get the skulls. You can't be playing easy.

This is the video for golden skulls by madhead360

Golden Skulls:

Iron

Black Eye

Tough Luck

Catch

Fog

Famine

Thunderstorm

Tilt

Mythic

Here is the one for silver skulls also by madhead360

Silver Skulls:

Blind

Grunt Birthday Party

Cowbell

IWHBYD