Umm. Guessing like, you already pushed campaign map too much.
If the campaign map still with IA armies as opponents, then have slight chance. Difficult from BA city no doubt, but having (huge BA army) troops in hand sometimes luckily can win 3 or 4 battles. Like 8 spearfighters, 8 warriors or with combination of rogues with spearfighter and warriors. Difficult to predict which combination works out, also cannot tell auto battle or manual to push forward. It works with auto-battle too(sometimes). Since under BA city, losing troops is not a big worry. Try to see any luck.
Tough to get 6 rogues, if available, try with 6 rogues and 1 spear fighter and 1 warrior. No guarantee, but trying in the game gives little tense and thrill moment.
If provinces are advanced above IA, then all the doors are closed to try anything. Only feeling is to get better luck next time.