As horde I found a rather easy way of farming coins solo (this applies only for ranged classes). Just stand behind the rocks in the water near the flight master and kill people who arrive on the island. Since it takes a while to get there I found that many people are AFK when going there, so they don't put up much of a fight. If you are a class with water walking it makes it even easier to escape if you get attacked. You can also use Cursed Swabby Helmet for 200% swim speed for an even easier escape.
By using this method I've farmed around 250 coins without spending much time at all.