What I do is focus on one ninja that I fancy at a time then get him when he shows up but power up when he don't. For example, Minato[jonin]. When he shows up, I use all my coupons and try to get as many fragments as possible but when he is not out, I just save my coupons and use the free event points for power up items. I saw my balance while doing this and I get new ninjas from time to time without compromising power.