Fastest way to earn coins?

I say do Survival mode. That helped me the most. And when you do that walk in a circle and kill the creatures! That helped me the most to level and get coins easy.

I recommend you play at least once of every gamemode once a day and win so you get double rewards and XP, when you level up you get some coins and gems more as you level up. Also spend 270 coins to make a gem creator you get one gem a day then make a coin maker to get 3 coins a day for a cost of 85 gems. Also play a lot of arena rounds to get money

What I do is play every single gamemode for x2 reward and play campaign. You get 2 times more experience and coins. Also, play arena. Get the lucky chests and open novice chests. They give 10 coins if you land on it, which is a pretty high possibility. Also spend 270 coins to make a gem creator you get one gem a day then make a coin maker to get 3 coins a day for a cost of 85 gems. They will earn you a fortune someday.

There are over 7+ ways you get coins, here is a video on the best ways (no hacks, because a lot of websites mention using cheats) to get coins...

1. Chests.
   Chests in the game have a chance of coins inside, and you can earn 10 - 250 depending on how many times you spin it.

2. Battle Pass Rewards.
   Completing battle pass quests gain access to rewards, and coins are guaranteed collectable.

3. Ads.
   Might sound like a cash grab, but the game gives you 10 - 50 coins for just watching 30 second ads, to gems.

As mentioned earlier, the cheat scamming won't get you anywhere, it is just an upsell and just warning you that you shouldn't use it, especially with the "no human verification" sign.