Arena will make you stronger everyday even if you have a bad rank you'll always earn rewards, you have 5 chances everyday of fighting then you will have to use diamonds if you have enough use them until you get 20 points, points will be described later.
In Arena you'll have to fight against real player. If you win you'll get higher on the rank and if you lose you'll not go down. You can everyday "Worship" the top 10 player and you'll get 100 000 Gold (10 000 per player).
When you win or lose against a player you earn 2 points. Those points will make you earn rewards, at 20 points you'll get great rewards. Those rewards are a "General Shards", 100 Fame Point (that are different form the point that you earn every battle) and 30 000 Golds.
At 9PM (UTC-5) the ranked will earn rewards that's depends from your rank at this time for exemple at my rank (between 301 and 400) I'll earn 340 Fames points (I'll talk about it in the Shops part), 85 diamonds and 22 000 golds, and more you'll be high in the rank, more you'll have better rewards.