I'm having issues snagging the Iron skull in 'Pillar of Autumn', as well as the terminal. The skull behind the waterfall in 'Halo' isn't there either. I've only found the Boom skull and the terminal in the 'Halo' level. I've seen gamers say that you need to have your difficulty on normal or higher, and my difficulty is on normal. Any ideas what could be the problem as to why these skulls and terminals aren't present?
Also, I might need a gamer friend who will be willing to play for the co-op achievements. I'm still thinking about it, but I will let you guys know ASAP. Thanks!