While this can be considered a PTW it's really not too bad. I only spent $5 for Garra who I don't even use and I'm #8 on my server in power/rank with only around 4-6 people I can't beat 100% of the time. My server has like 5 people that spent thousands on the game and that was weeks ago, they spent even more by now.
If you spend the time doing everything you can daily even FTP players can compete with paying players.
As for coupons we get a decent amount...We get another 20 a day from the puzzle event. I get 100 a week from ninetails, 130 a week from my lvl 6 tree, log in reward is another 250 a month we can get 10/20 from lucky wheel, when you set a record in group trial you get 25/50 AND you get coupons when you get a new ninja!
All of the good ninja like the pains will be in the shops at some point and they are only 3* for now. A team of more common ninja at 5* have a nice advantage over them. If you did not need the low level refine runes and other stuff a FTP should be able to buy like 30 pain fragments from the event that just ended which would save time buying him from the shop when he gets added.