Use Fire Spirit against small cards like Goblins, Skeletons or Bats; its explosion hits them all at once. On offense it clears small defenders in front of a tank. In cycle decks it provides cheap splash damage.
Tips
It's a cheap answer to your opponent's Princess or Dart Goblin.
Aim it at the middle of a swarm so the explosion hits everything.
It also hits air, so it helps against Minions and Bats.
Strengths
Very cheap splash damage.
Hits air and ground.
Weaknesses
One-time use with little effect on big troops.
Easy for your opponent to answer with a cheap card.