It is a spell that repeats your last card played for 1 extra Elixir, as the description states. The level of the copied card depends on the level of the Mirror.
When in a battle, the card will display your last placed card inside the Mirror.
A Mirror card doesn't have a defined Elixir cost; instead, it shows a "?".
At Max level the Mirror will place a card one level higher than your previous card's level. This allows for level 14 Commons, level 12 Rares, level 9 Epics and level 6 Legendaries!
The Mirror lets the player repeat a powerful card, such as the Goblin Barrel, leaving the opponent without Elixir to counter it.
There are a few ways around it though, since the opponent can have one card that counters both, like a splash troop, and Elixir generates at the same rate, so the Mirror user needs an Elixir advantage.
It is also possible to mirror a card which you know your opponent only has one counter to. For example, if you notice your opponent always counters your Goblin Barrel with the Log, you can mirror your Goblin Barrel and your opponent will have no Log to counter it.
Elixir Collectors can be mirrored in order to boost the player's Elixir production, allowing them to deploy cards faster, although it will give a profit of only 1 Elixir.
It is easy to tell that an opposing player has the Mirror, as it is the only way to deploy a card twice in a row, or have two of the same card at different levels on the field.
If the player mirrors a Sparky it will often deal heavy damage to the enemy, especially if they surprise the opponent. It does cost a heavy amount of Elixir, though, and can still be countered relatively easily if the opponent is experienced.
The Mirror is a high-risk, high-reward card. If played correctly using powerful cards like the Giant Skeleton or P.E.K.K.A., you can very easily get a three-crown victory.
This strategy is especially effective during the double-Elixir period, allowing you to mirror the above cards along with support, making it a nearly foolproof strategy.
You opponent will gain more of an Elixir advantage if your mirrored card was easily countered.
There are cards which aren't wise to mirror, such as:
Minions, as you can play the Minion Horde to get the same number of Minions with two fewer Elixir cost.
The Mirror can be used as a surprise card. Don't play it until the right time comes. That way you can surprise your opponent if they aren't prepared for twice the amount they were going to defend for.
The Mirror can be used along a Clone spell to make a large push and overwhelm your opponent (with a high-damaging troop or low-health troops), if he/she can't defend against it. This tactic is not advisable to use with low health troops, as splash damage spells can take down those troops with ease.
The Mirror was added with Clash Royale's soft launch on 4/1/16.
On 2/2/16, the February 2016 Update increased the level of mirrored common cards by 4 and mirrored rare cards by 2.
On 23/3/16, a Balance Update decreased the mirrored Legendary cards level by 1.
On 3/5/16, the May 2016 Update increased the mirrored Common and Rare cards level by 1.
On 19/9/16, a Balance Update increased the mirrored cards level by 1. This allows players with level 8 Mirrors to mirror cards 1 level higher than max level, which would be unobtainable otherwise.
The Mirror is the only card that doesn't have a defined Elixir cost.
When upgrading it, the Mirror will show its old texture.
It is the only spell that does not have any direct effect on deployed troops and has no area of effect.
When calculating the Average Elixir Cost of a deck, the Mirror is treated as a 2 Elixir card. Before the May 2016 Update, it was simply ignored in calculating the average cost as if the deck had only 7 cards.
The Mirror can produce cards at levels otherwise unavailable such as level 14 commons when it is at max level.
The mirrored card image will be reflected in the Mirror, only to be looking a bit faded.