you cant get the Armour on co-op
To unlock the EVA “Body”, you will need to complete “Tsavo Highway” in Campaign mode on either Normal, Heroic, or Legendary.
To unlock the EVA “Shoulders”, you need to complete “The Ark” in Campaign mode on either Normal, Heroic, or Legendary.
To unlock the EVA “Helmet”, you will need to complete the Campaign on either Normal, Heroic, or Legendary
also some skulls don't give you achievements. only certain ones will give you achievements i believe there the ones that are closer to the end of the game.
My gamertag is: DeeNize if you need help with the skulls.