Military command structures are strictly vertical; generally speaking, officers at any level do not command across ranks.
Even if Meng Tian had the ability to precisely command down to the battalion and company level, he would not easily bypass their superiors to issue direct orders.
More often than not, the central command headquarters merely issued combat orders, requiring the designated battalions and companies to complete assigned combat missions within a specified timeframe. As for the process, that was a matter for the post-battle merit officers, and it was also how superiors judged whether you were worthy.
Chapter 878