Unlocking skulls is done by completing certain side objectives during each mission in the campaign. A signal will go off on the mini radar where the skull has appeared on the level.

Ohhhhh i found one on one of the final missions randomly. now i just need to try harder on side missions!

the hardest skull to get is the first skull, which requires you to kill 100 grunts on the first level, because if you play on a difficulty lower than legendary, you'll never get the required grunts

I don't know what you are saying. I got all of my skulls on the easy difficulty.

at the least for the first mission skull, for it to be on easy you would have to turn on some other skulls that made enemies tougher and stronger, cause Grunts are so weak on easy that the machine gunner will kill them before you can run them over which is what makes the first skull so hard.

If your talking about the achievement for ramming 50 Grunts than yes you would. But killing 100 grunts is a whole different story. ;D

quote Zhents

the hardest skull to get is the first skull, which requires you to kill 100 grunts on the first level, because if you play on a difficulty lower than legendary, you'll never get the required grunts

i got that skull on heroic and it was easy to get