The number of really good ninjas you can buy for a reasonable price with coupons is pretty low so, after you got them, you can wholly focus on power and just gather fragments for free in events that let you do it. In my specific case, for example, i just bought with coupons gnw tenten (edo tensei treasure wasn't out at the time) and sakura sailor uniform and i wholly relied on seal scrolls and free draws to get competitive teams.