I was struggling with this for a while, but actually found this easier to accomplish during a Multiplayer session with the Practice AI.
I would suggest starting on the Battered Bluff (the first level), setting the AI at "Most Gentle" and picking the Drowning Doom as the opponent.
Begin by capturing all of the fan geysers except for the two that are in front of the opposing stage.
From this point, have all of your troops hold position just outside of the stage area so as to prevent enemy troops from reaching your Merch Booths.
Upgrade your stage to the point where you get Fire Barons.
What I found is that the AI will eventually start spawning nothing but Grave Digger foot soldiers in packs of six. Hop on the back of a Fire Baron and simply cruise around until there are three sets standing in front of the stage. Once this occurs, FLAME ON and take a wide circle around them.