but it never says "achievement unlocked" or watever so i don't want to do legendary for nothing.
You don't have to be on Legendary, just Normal or higher. Make sure you start the campaign level from the beginning. If you choose a checkpoint, it won't work correctly.
Also, what are called the 'Silver' skulls will not give you achievements, only the 'Gold' ones will.
As people said in previouse posts..only GOLD skulls will say achievement unlocked,not SILVER skulls....as u pick up the skull, do whatever u want With it and u will still have the skull -