While just willing yourself not to lose really doesn't matter, keeping disciplined and holding your line goes a long way though. Rome conquered most of the known world of the time on the back of discipline, logistics and engineering. The average Roman solider was actually not a particularly good warrior though. That really isn't what is needed to win ancient battles though.
Part of discipline is believing that you'll win. I think its harder to force a cohesive unit together that believes that failure is imminent. Its all part of the same thing as far as I can tell. Conviction, tactics, discipline, superior fighting strength (be it raw numbers or more talented troops, or both), they all play a crucial role. Without all of it working together, you're more likely to fail.
Granted, in the case of sranc, which have an infinite supply of bodies, if a host is stranded and engulfed on all sides, at some point that host will break. Fatigue should be very real for the men of the tusk. Without the ability to back peddle, relieve the front to rest out of harms reach, and prevent so many bodies form piling up that it forms a ramp over the front lines, they would be slaughtered. This is whats so important about discovering the ten-yoke army early - had they not, and been surrounded, all would have been lost.
Given a large enough host, they might have a small enough surface area to defend, they might have enough time and space to fully rest troops before they have to go back to the front. But even then, assuming the sranc never stop, at some point their swords/shields/armor would break or be lost faster than they could be repaired or replaced.
Regardless of most of that, schoolmen are the real key. If they could stay safe and sing forever, or sing in concerts for long enough for fully rested replacements to arrive, the battle would remain in a stalemate. Then the battle would last so long that you'd have to start worrying about food. It would be like a continuous siege. Something, at some point, would give.
Good thing there isn't an infinite number of sranc, and they do run away at some point. Shackled to the will of the No-God though... ill tidings.