If you want all the achievements, then play Legendary. If you just want the "Complete Mission #" achievements and the skulls, play on anything other than Easy.
No, the skulls are found in the campaign levels on the normal difficulty or higher
On campaign on halo 3 (difficulty normal or above) by the river on the first level jutting out on a rock theres a skull.
on the campaign menu press X
Go to the campaign menu and press X and your skulls collection will pop up.
I am not sure if skulls are on the screen means that you have beat the campaign. It usually just means that you have beat the mission.
Play halo 4 no one gives a $hit about halo 2
Different skulls are available on each level. You must be playing on Legendary difficulty for them to appear.
You have to go to campaign and press x.
find all 13 campaign skulls
You have to find all the skulls in campaign mode.
press the X button on the campaign selection menu.
Completing missions in Campaign,playing online, the rest you can find out if you hold down the giant X button in the middle of the controller then going left until you see ''Acheivements'' and see Halo 3 Acheivements.