Fire Spirit is great for punishing swarm troops like Skeletons or Minion Hordes with one tap.
It combos well with Hog Rider or Battle Ram to clear small counters before the main push arrives.
Place it in front of your win condition to ensure deathball efficiency at low cost.
It performs well in Cycle decks wanting cheap and quick value trades.
Since it is a one time use troop it can be baited easily so alternate your cycle.
It works as a defensive poke and helps force out reactions from your opponent.
Understanding how much damage it deals to various level common cards matters for value plays.