You could be unlucky like me and get the coins from the group lucky wheel every time.

It's either focusing on strengthening or group skills first - I prioritized on group skills over strengthening. After maxing out my group skills, I focused on the latter.

Before doing that, however, I strengthened my gear to at least a certain amount and left it there just so I wouldn't fall behind.

Best way to get coins, other than through rich fields and survival trial, is through killing the 9 tails everyday.

Get as high damage in the World Boss.

http://forum.naruto.oasgames.com/en/forum.php?mod=viewthread&tid=1496&fromuid=20658

if you have the time to manually do instances, you can get anywhere from 4 to 21 times the amount you auto depending on the instance. Its boring though.

im always getting frustrated with people in my group that waste all their coins on strenghtening even tho its not worth it until you donate max coins everyday,dont bother much with strenghtening,prioritize stuff like bm and group first then see if you have enough for strenghten too
I get coins from 9tails,survival trial,lucky wheel,from arena shop too card flip(got 50k coins one from summoning lol) and events,facebook comunity share too,arena grade,and home that would be all the places you can get coins from oh and yeah coins trial,sry for long post^^

How many coins u can get everyday?

Once your server can take down 9tails daily, getting more coins is so much easier. Currently, I get around 2 millions coins per day from survival and 9 tails.