I have been trying to get a Legendary Card in chests given in arenas, but it seems to be impossible. Can someone help me figure this out?
In the arena, you can get legendary cards from mega lightning chests, but they are very rare. To check on it, click the info button from the arena and you will see the chests.
If you get a legendary chest, you will definitely get a legendary. Also, super magical chests have a very high chance of getting a legendary (with more cards too!).