Gold skulls are awarded for completing meta goals in the game (such as beating the first level while taking no casualties.) You may earn some of these while completing your Normal campaign, but don't worry about them. You can do a second playthrough on Casual focusing on the skulls using level select.
There are a total of 55 gold skulls available, and you will need to earn 15 of them in any order to unlock this achievement.
Shelton's very detailed guide to earning all 55 gold skulls is available here, with both text and video for every skull.
*** Note that this achievement can become permanently unobtainable on some devices if you unlock it on one without doing the simultaneous kill to end the level on the other devices. For instance, if you play on the WP and unlock this achievement, and then later complete a level on the Xbox, for many people the achievement will not unlock on the Xbox even if playing offline on a clean install. This glitch applies to all of the campaign achievements!