Rare Card: The Furnace
If saving up elixir is concerning for you, then you are a player that prefers to play with spawners. You do get much of the value when playing of such strategy because instead of investing on units for a single push, you get multiple troops over time. The Furnace is one of the spawner cards out there. Deploying this allows you to get a pair of Fire Spirits every ten seconds. The thing about spawners is that you do not get control over them, which means, once they are released, they automatically go off to fight.
A good strategy when using spawners is to use other spawners as well. For example, you have placed the Furnace, and then if you place a Barbarian Hut beside it, that would be a great pair as those Fire Spirits have very low health points. You need to back them up with a tank or else, they would literally be toasted! They might appear to be adorable but they are quite deadly.
Double the elixir cost of the Fire Spirits and you can get yourself a Furnace, which is a great investment. This is because of the fact that you can summon 12 spirits before it wears off. Of course you have to keep it safe in order for you to produce such amount, as this structure, same with the other spawners, can be damaged as well. You have to ward off those long ranged snipers and of course those pesky spells with Guards.
If you are the offensive type of player, then you should aim for the double furnace, which is basically a mirrored furnace. This gives you 4 Fire Spirits every ten seconds, and if you pair them up with a massive tank such as the Golem, it would be a lethal combo. No one would dare step passed these angry mob of flame and stone. They can rip through any horde that will come against them. However, the Fire Spirits does not do much damage to the crown towers so you have to build up your elixir to pair the combo with a princess or any other long ranged attacker to finish the job.
You cannot rely mainly on the furnace alone as it works more of a defensive/supportive type of card in the game. There are lots of attackers that you can mix and match though. It just takes a matter of practice as you need to get used to the different encounters in Clash Royale.
The Furnace can be acquired as soon as you reach Arena 5, which is known as the Spell Valley.