Getting Crystal Coins in Clash Mini
You can get Crystal Coins from special quests, the Season Road, and from getting copies of units that you already have maxed out. Quests won't appear super often, but keep an eye for the ones that award Crystal Coins. The Season Road is one of the best ways to start getting Challenge Coins, mainly due to getting so many chests that you can unlock and max out more units. The primary source of the coins comes from collecting extra copies of maxed out units. They are converted at a 1:1 ratio, so if you have Shield Maiden maxed out and get 1 copy of Shield Maiden, you'll get 1 Crystal Coin.
Another way to get Crystal Coins is when a Season ends. At the end of seasons, you can optionally choose to reset your Minis. When doing so, the game will accumulate the total number of stars you've earned across all your Minis and the more you have to reset, the greater your reward. One of the rewards from this is Crystal Coins. Check out our Should I Reset My Minis? page for more information on the rewards for resets.