I keep getting overwhelmed by the enemy team. My AI teammate is of no apparent use. How have you guys and gals beat this level?

run a stack into the big boni and then wipe the floor.

1st turn: 1st move, take podezeno territory, if your AI partner takes it before you, restart the level. Take the other 2 territories of the bonus. Have two stacks take the carpento territory
2nd turn: Complete the bonus by taking the last area (use combined stack from you and AI to take vigolzone). Have you final order take Ponte del olleio
3rd turn onwards: Focus on eliminating AI 4, then AI 3 and deal with AI 2. If you manage to get lucky and your AI partner breaks AI 2's bonus before you get to it (makes it easier to net a gold star), deal with AI 2 and 3 simultaneously.

Hope this helps

Perfect!