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.
I think there's a way to have it work with multiple people. The benefit might apply as long as you and your party members are NOT engaged in the SAME combat encounter at once. If that happens, or an ally joins the fight, you lose the buff immediately.
