[wulfir]

This is what I would do:

1) Make the scenario a player 1 advance or assault mission.
2) Give the AI forces the majority of the Victory Hexes.
3) Buy the defending AI forces and deploy them.
4) Change the scenario into a meeting engagement.
5) Buy the AI forces you want to advance on Player 1 and deploy them.
6) Set reaction turn on the AI forces you want to attack. Assign waypoints. Maybe reinforcement turns.
7) Change the scenario back to Player 1 advance or assault.
8) Fix the player 1 units.
9) Profit

(Note: when the human player has successfully captured enough VHs the AI will counterattack with its mobile units, regardless of reaction turn.)