When I do the multi battles I get copper, silver and gold coins as reward. I do not know what to do with them other than sell them. do they serve any other purpose?
They aren't very useful, unless you have a hard time with coop and need extra pots, revive, and ougi buff. I pretty much sell all of the coins. Actually I sell everything from coop besides the 4 leaf thing. I like to keep 4 of those for drops buffs.
Not sure if any of the mods still support this community, but they should be aware that the forums are infested with bots and that their forum needs to be upgraded.