Zheng was strong both physically and mentally after surviving countless life and death battles, enduring through the deaths of many comrades, and overcoming his heart's devil. He had nearly patched up his flaws in every aspect. He wasn't strong enough to completely disregard other teams yet, but he could be crowned as non-human by the other teams.
Still, Zheng wasn't omnipotent.
A hero alone could not carry a team through God's realm. Individual heroism usually equated to idiocy in the realm and could only get by against ordinary teams. Any team with a decent strategist on the level of Neos, or perhaps even Julian could kill this heroism along with the person's team. The only exception was when this team's strength achieved a limit where wit no longer had any effect on it, just like the current team Devil.
A team, as it suggested, required talents from multiple fields. The strategist, the psyche force user, close and long-range combatants, researchers, medics, etc… A complete team required the cooperation of too many people. Team China was a rare case where they had a variety of talents, including the researcher who tended to have the least self-sustain. Furthermore, their researcher was also a strategist with unfathomable strength. A lot of times, he gave off a sense of being stronger than Zheng.