Zeras plan was to appear so powerful that he alone would be enough to cast fear and doubt into the hearts of the opposing army.
He was to make everyone look so powerful by going absolutely all out, causing the devils to completely forget about their own large number advantage and instead believe they were fighting a losing war.
He was to make himself appear unstoppable and unbeatable, casting doubt and fear, which he knew would cause the teamwork of the devils to go haywire as their hearts shrunk in fear of being the next target. And with him making everything look so easy, it would further fuel his own army, who would believe they were winning.
As a result, their movements would become more coordinated, their initial doubt fading away as they fought with even more power and cohesion, gaining the advantage over the enemy.