Halo 3, Skulls, Difficulty?

[airdogspace]

I’m going back through Halo 3 again to get all the Skulls. Do you have to play it as Legendary to get some of the skulls? (I know some you get and unlock on normal settings) but I thought some of them you have to be on legendary.
And there is just 13 skulls right?

 

[Michael C]

http://www.halo3-skulls.com/halo-3-skulls