To deal with big swarms, you must place him at a good range since he has a low fire rate.
The Wizard is very good at taking down Balloons due to his high damage per second.
When defending against a Wizard that has locked on to your tower, do not place your unit behind the tower as the Wizard's splash radius will splash the unit as well.
Due to their low amount of hitpoints, Wizards are very vulnerable to moderate to high damaging spell cards such as the Fireball.
A higher level Fireball will always destroy a Wizard.
When trying to counter a Wizard, deploy moderate to high hitpoint troops to eliminate him, such as the Mini P.E.K.K.A. or a Knight.
The Wizard deals moderate area damage, so it is not advised to use multiple-unit cards such as Minions or Barbarians unless it is absolutely necessary since your troops will be destroyed or critically weakened, allowing the opponent to take the better end of the trade.
Despite having area damage, it is still possible to eliminate a Wizard by surrounding him with troops with low or moderate hitpoints such as Barbarians or even Skeletons due to his relatively small splash radius.
The Wizard is very similar to a Bomber, so use him in most situations where you would use a Bomber, plus with air troops.
However, surrounding the Bomber with low or moderate hitpoint troops is not advised, as the Bomber has a bigger splash area than Wizard, and will be able to hit most or all of the troops you surround it with.
One cheap way to counter the Wizard is the Musketeer due to her fast hit speed and decent damage, making a 1 Elixir profit easily. An equal level Wizard will leave the Musketeer with a sliver of health left, enough to do some damage if a tank is placed in front.
The Wizard pairs excellently with the Tornado, as the Tornado sucks troops into the middle while the Wizard splashes the units in the center of the tornado.
The Wizard is a great counter against the Lava Hound since he has high damage per second and deals with swarms (the Lava Pups). If timed perfectly, the Wizard's fireball will launch before the pups have a chance to spread out, killing all of them before they can do any damage.
However, the chances of this occurring are very low.
The Wizard was added with Clash Royale's soft launch on 4/1/16.