Just as Swen said, One-man-army is planned to be available from level 1 on. But at the current state if would be totally game breaking for single player or I guess even duo-coop, because the benefit is to strong, if you can take it with every player char and companion.
Also still not sure, why single player is seen as that much harder. At least fighting can be much easier, because you don't have to coordinate tactics with other peoples. With 4 people that surely can be tough.
