Thought I'd finally gotten my hands on a decent campaing farmer, unfortunately her skill choice is a bit weird on 2nd wave, probably due to the cooldown reduction mechanic on her A3:

1st wave - Uses A3 as expected, wave dies, cooldown reduce -4 (only need 3 to have it available next turn)

2nd wave - Uses A2

3rd wave - Uses A3

Seems the AI should use the A3 every time since the situation hasn't changed, 4 enemies, priority should be AoE.

Note you can override this by using preset priority, but this isn't really an option for campaign farming when you're swapping food in and out constantly.