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How to get legendary cards in clash royale 
Buy legendary king's chests in the shop. 
It will cost 500 gems for a guaranteed legendary. This is one of the risk-free methods as you have a 100% chance of getting a legendary.
If you save up enough gems, you can also buy it without having to spend money on it. Legendary chests are best when you only have a few legendaries as you have a smaller chance of getting a repeat of one you already have.