It is a powerful spell, available to cast anywhere in the Arena, dealing immense damage.
The Fireball will push back certain small troops.
The Fireball should be used to used to finish off weakened Crown Towers or buildings or killing/crippling large numbers of small troops like Barbarians, Spear Goblins, or Archers. However, it is unadvised to Fireball high hitpoint troops like a Giant or Valkyrie alone as it will hardly affect them and results in an Elixir waste. But a Fireball may be used to finish off "low health" tanks.
For low-level players, it's recommended to use Lightning instead of Fireball, because of its higher damage to Crown Towers and individual troops; but if it is more suited to deal damage in an Area of Effect, damaging many troops and Buildings, the Fireball is recommended. (The Lightning can only damage 3 targets.) However, Lightning costs 2 Elixir more than a Fireball.
Wizards, Musketeers, Ice Wizards, Witches, and Barbarians can typically survive the Fireball. A tank in front of them might be a disaster as the surviving troops can plunder the player's tower. This can be avoided with a Fireball and Zap or The Log combo by quick-dropping Fireball and Zap. One thing that is important to know is that the Fireball pushes back units, so the player may want to time it properly or the Zap might be missed. A Zap-Fireball is safer.
If the player is using Fireball, be patient. If the player waits for as long as they can, they may get an even bigger Elixir advantage than if the Fireball had been used before.
If the opponent repeatedly deploys Barbarians or other troops that are weak to Fireballs like Three Musketeers in the same place, a high-risk, high-reward move is to launch the Fireball where the troops have been planted. This can backfire if the opponent does not have enough Elixir to deploy their card in the first place, though. Do this only if you know your opponent has enough Elixir and the card is back in their rotation.
It is effective against spawner cards because they will lose health and thus spawn fewer troops. However, hitting only one spawner will result in a negative trade as the spawner will typically still have enough health to make many more waves.
The Fireball can be effective against Elixir Collectors, as it will lower its hitpoints enough to make an equal Elixir trade, which can become very beneficial if it also hits other troops or buildings in the blast.
If your opponent places a small troop close to the center of the arena, such as a Wizard, a properly placed Fireball can knock it back so much it heads 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.
The Fireball was released with Clash Royale's soft launch on 4/1/16.
On 19/2/16, a Balance Update decreased the Fireball's Crown Tower damage by 20%.
It is one of only three Rare spells, the others being Rocket and Heal.
The Fireball's description is a reference to a Clash of Clans commercial, Magic.
The Fireball and Arrows share similarities — the Fireball inflicts more damage but has a smaller AoE (Area of Effect) than Arrows. Also the Fireball has a pushback effect, while Arrows do not move the targets.
Fireball deals the same damage as an equal level Mini P.E.K.K.A.; thus, it can take down a full-health Wizard or Musketeer at least 1 level lower than it in one shot.