“Mirrors your last card played for +1 Elixir. Will not appear in your starting cards.”
The Mirror card is unlocked from the Electro Valley (Arena 11). 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 maximum level, 14.
The Mirror lets the player repeat a 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 the player to deploy cards faster, although it will only give a profit of 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.
In 2v2, it's also possible that two players from the same team use the same card of different levels.
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 13 Elixir though, and can still be countered relatively easily if the opponent is experienced, like with their own Sparky.
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 win.
This strategy is especially effective during the double-Elixir period, allowing you to mirror the above cards along with supporting them.
Your opponent will gain more of an Elixir advantage if your mirrored card was easily countered.
If you accidentally play a card too early or otherwise at the wrong time, you can use the Mirror to play that same card again but at the cost of 1 additional Elixir.
There are cards that aren't wise to mirror:
Minions, as you can play the Minion Horde to get the same number of Minions for two less Elixir.
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 to make a large push and overwhelm your opponent (with a high-damaging troop or low-health troops) if they can't defend against it. This tactic is not advised with low health troops, as splash damage can take down those troops with ease.
The Mirror can produce cards at levels that are otherwise unavailable, such as level 14 cards, when it is at max level. Additionally, it allows the use of cards above tournament level such as level 10 cards when played at tournament level. This one level difference can change a lot of interactions between cards, assuming they are at similar level. For instance, a mirrored Fireball will be able to destroy Three Musketeers outright.
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 2/29/16, the artwork of the card was changed.
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.
On 20/6/18, a Balance Update changed the Mirror such that, if used in a deck, will never appear as one of the cards in the player's opening hand. It also moved the Arena to unlock this card from Spell Valley (Arena 5) to Electro Valley (Arena 11).
The Mirror is the only card that doesn't have a defined Elixir cost.
It is the only spell that does not have any direct effect on deployed troops and has no area of effect if you are not copying a spell.
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. However, the Mirror is now listed first when sorting Cards by Cost in-game.