The Frontier Pyro appears in battles against Civilians, where the player must use Apprehending to win these encounters. They provide the Angry Civilians with indirect fire support. What they lack in damage, they gain in the chance to stun the player's army, disrupting the best laid out plans.
Some mission-based encounters will include some friendly Frontier Pyros. Note that these are the Civilian-versions and will not earn for the Frontier Pyro (player)-version after these battles.