Probably group skills, a lot of people neglect them but they are much better then gear strengthening.

Well if we determined the problem, stop spending on strengthening and try to ease up on black market, only buy materials and frags for ninjas you really need and focus all your coins into group skills.

As for where to get a lot of coins, full clear survival every day, do rich fields whenever it's up and do 9 tails with a proper team, 9 tails gives so much coins, especially if you are on an emptier server so it lives for more then 5 minutes lol.