Master Chief's Armor: You will find this on the level Vegas in Ruin. At the start of the mission you will find a soldier standing next to the delivery truck. A joke will come about that he is apart of, it will be about the Green Master Chief armor, jump into the truck and you will see what they are joking about, obviously, this is the Armor you are looking for.

Issac Clarkes Helmet: This will be found on The Hive Duke mission. A character will become to comment about this certain door, that has a fleshy look. You will start passing through and you will turn left into the hidden area, after this Duke will quote about the dead space marine, and you should come across the helmet.

Borderlands Psycho Head: This is found on the mission Blowin' the Dam. After you swim past the guy with the concrete shoes, you will go along to the next airvent and you will see a corpse that will have a torch. You will swim toward the light and eventually to the left towards a disk like structure that will have an opening at the bottom. Swim into the structure from the opening and from there you will find the head for the achievement.