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“Annnnnd... Fireball. Incinerates a small area, dealing high damage. Reduced damage to Crown Towers.”
The Fireball card is unlocked from the Training Camp (Tutorial). It is an area damage spell with a medium radius and moderate-high damage. A Fireball card costs 4 Elixir to cast. The Fireball will push back certain small troops.
As a general rule when using the Fireball, be patient. If waiting to use the Fireball on a target, wait until another unit is deployed next to it to get an even bigger Elixir advantage than if the Fireball had been used before.
The Fireball should be used to finish off weakened units or to kill/cripple large numbers of small troops like Barbarians, Spear Goblins or Archers. However, it is unwise to use a Fireball on high hitpoint troops like a Giant or a Valkyrie alone as it will hardly affect them and will also result in a negative value trade.
Wizards, Musketeers, Ice Wizards, Electro Wizards, Witches, Mega Minions and Barbarians can typically survive the Fireball. A tank in front of them might become a disaster as the surviving troops can quickly take the player's tower. This can be avoided with a Zap or The Log after the Fireball. One thing that is important to know is that the Fireball pushes back certain units, so the player may want to time it properly or the second spell might be missed.
If the level of the Fireball is higher than that of an equivalently leveled Wizard, Electro Wizard or Musketeer, it will destroy them in one hit. This could result in a quick positive Elixir trade in the case of the first.
Remember to destroy the tank after everything behind it is gone. Otherwise it could do heavy damage to or even destroy a Crown Tower.
It is effective against spawner buildings such as the Goblin Hut because they will lose health and thus be denied a few waves. However, hitting only one spawner will often result in a negative value trade as the spawner will typically still have enough health to spawn a few waves.
The Fireball can be effective against Elixir Collectors, as it will lower its hitpoints enough to deny a lot of Elixir from being produced for a positive Elixir trade.
If the opponent places a small troop close to the center of the arena, such as a Wizard, a properly placed Fireball can knock it back and make it head the other way after recovering from the blast. This can be useful for diverting small troops to the other path, especially if that path is better defended. This should only be done with a Fireball if Tornado is not being used or is out of rotation, as Tornado is much more reliable for doing this.
In Overtime, a Fireball can be used to quickly take out an enemy Tower and secure a win if the enemy's Tower has low enough health.