If you have patience, save up to 20K to 40K coupons to spend when Fukurokumaru's Deals is available in Hot Topics (it appears this week). Spend in events where you are able to redeem ninja fragments or power items where it is guarantee to get.

Do not spend in RNG events like Lucky Stars or Lucky Sn@tch with the purpose in getting the Jackpot prize (80 ninja frags). You can spend if you have enough to get something if return after spending the amount of coupons you have. So if you have enough coupons to complete a ninja in lucky stars, try to complete it there as you might get another ninja as well if you are lucky.