If I am not mistaken, some of them are locked and require a co-op partner, but for a few of them it would be better to have a partner. One that easily comes to mind is the killing Juggies with explosives and knives. Could just get a friend and camp the lil one way room at the far east of where you start. Have a guy prone by the door, and the other hiding behind the stove crouched. Dun really remember tho, been a while since I got the Star 69, and have been itching for more spec ops in the form of DLC, but can really only hope for such things.