In that case then either the numbers are equal ( if you also attack with one army with full stack) or the enemy outnumbers you slightly ( by 30 to 50% ) if it has his full garrison army there as well. Is it practically impossible to achieve a decisive victory there? And even a close victory? Is a pyrrhic victory the most likely result of the battle in case you're going to win?
Some might say "Why don't you first besiege the city to make them sally out after several turns? of course that's the ideal thing to make sure you reduce the amount of losses in your army. But I would do that only in case I have all the time. I won't wait for them to sally out specially if it's very likely another army would arrive as reinforcements: In this case I would be in bigger trouble because the numbers would be heavily against me.