Lava Monster isn’t of much use early in the game, but perfect for sticky situations in the late game. With raw stats and a high cost, it’s a pretty stereotypical card, save for one thing-- pay 3 energy and it gains +1AP and +1HP! This can be done an unlimited number of times, so if you keep going Lava Monster can become a frightening beast (more than it already is!). Since in the late game energy is high and there aren’t many cards to spend it on, there’s no longer any need to worry about wasting energy. Just spend everything left over on growing your Lava Monster! Be careful, though-- if the opponent is playing a black deck, they may have either Engorger or Frighten to Death; this can lead to you fighting off your own Lava Monster, putting you at a great disadvantage.
Strong against: lack of allies in the late game, decks with low-ally count (unless they have powerful ally removal actions).