None of them where really hard for me but i can understand why people have trouble with the bandanna skull the only reason i got it was because iv done allot of grenade jumping. My problem wasn't that they where hard or anything though my problem was that it feels like they where lazy hiding them like in Halo 3 i think it took nearly a year for people to find the IWBYD skull but in CEA i found like half of them myself but other than that great game.