I just thought of something else! There is a trick that it took me quite a bit of practice to work out. Ready?
Just because it is your turn to move first (most often) doesn't mean you have to move your units. Just click on them where they are or sometimes move them to the sides if they are in a first rank so they actually move back one space. That often (not always) will keep them out of range when the AI makes it's first moves. It is actually very often to your advantage to not go first. This way, especially if you have ranged units like slingers or archers, the enemy moves first so that when you advance into the field, they are then in range.
It's hard sometimes to not want to move onto the field right away. It takes practice and it takes patience to figure out what is the best strategy for each unit and each landscape. I was really bad at this kind of warfare (taking turns) when I started. I learned a lot in our empty neighborhood by repeatedly taking on the untended cities with just two spears. Working your way up from the bottom allows you to gradually face more challenge. Practice helps a lot.
The first time I faced that challenge, I ended up negotiating even though my neighborhood (at the time) barely had two coins to rub together and no one seemed to have the bonus materials. The second time around, I was well practiced and made short work of the balista and other units with my stone throwers and slingers.
Good luck and good fun to you.