Perhaps, it's possible for a BG Tank-like War (and for balanced 2H Wars too) to complete The Gauntlet without an ally at Nightmare level for now.
But reward in this case may be lowered...
You may try to start The Gauntlet with a friend-ally and finish it with an ally (or something like that) to raise rewards quality. Because there is some ... feature which allows to keep a starting friend-bonus for a whole run.

But it will eat something about months or more anyway...