You could use the battle editior to set up battles like that. More time consuming but lets you set better battles.
EDIT.
To use the battle editor
1, Open up the properties tab for your RTW shortcut...
2, Find the target line in properties...
3, Go to the end of the Target line
4, Insert -enable_editor (remember the space infront of the -)
5, Check if the editor option is avaliable in your Rome: Total War menu (from memory it could be in Options)
6, If it doesn't show up post back in this thread and I'll try and work out why (screenshots of the target line and stuff in your RTW shortcut would help)
[This message has been edited by Roman Warlord (edited 11-05-2008 @ 10:28 AM).]