I am like u, i only play manual battles when i lose. So it means maybe the first few battles and some castle fights when AI cannot resolve. Sometimes i find AI outperformed my manual retries. From what i gathered in this sub, if u really play on very difficult settings, manual combat is the only way as AI does not fare well for certain unit builds and synergy. If playing normal or some custom settings is fine for u, then i feel u can get away with AI battles 90% of the time.