The Inferno Tower is the best anti-tank building card in Clash Royale. It deals insane amount of damage overtime. It targets one troop and slowly builds up damage, which counters any high health troop. It targets both ground and flying units. It is mostly used to counter Giants, Royal Giants, Golem and Lava Hound.
The biggest weakness of the Inferno Tower is when facing multiple enemy troops (swarm cards like Minion Horde, Skeletons, Goblins, Barbarians, etc). It is important to have damaging spell cards like Fireball, Arrow or Zap to use in combination with the Inferno Tower.